Sentient Gelatinous Cubes can't be forced prone or deafened.

Your Constitution score is increased by 2, and your Strength score by 1.

Sentient Gelatinous Cubes don't visibly age, they can be anywhere between hundreds or only a few years old. 

Sentient Gelatinous Cubes can be any alignment, for who truly knows the heart of a cube? (Not that they have hearts.)

Although normal Gelatinous Cubes are very large and grow as they consume, the Sentient Gelatinous Cube is smaller and does not grow as they eat. Your size is Medium. Unless you are conforming to fit into armor or clothing you take up your full space.

Your base walking speed is 15 feet.

You can speak, read and write Undercommon, Abyssal, and one extra language. While a Cube may gain it's Sentience from a variety of sources, all oozes can trace their lineage back to the demon lord Juiblex and the Underdark. This ancestry, and possibly the consumption of a number of intelligent creatures and their books, has given the Sentient Gelatinous Cube knowledge of Undercommon, Abyssal, and at least one other language. 

Most of the Sentient Gelatinous Cube's body is clear ooze, except for its brain and eyes. By holding very still you can attempt to hide even without cover. While attempting to hide when only lightly obscured you gain advantage on any stealth checks. Any items the Cube might be holding move to the ground or move into any area within its space where they can be obscured. This effect is unusable in the Rain or Snow, and can only be used while not wearing armor or clothing.

Though Sentient Gelatinous Cubes are typically Cube shaped, you do have some control over your shape. The Cube form is the most standard and restful of all shapes, but you can conform to the inside of armor and clothing that humanoid creatures typically wear, or even squeeze yourself through small openings. 

Conforming to non-Cube shapes is stressful and cannot be done on a continuous basis. Sentient Gelatinous Cubes can only conform to a non-Cube shape for up to 16 hours, before then needing a long rest. Though short rests can extend this time by a few hours. If a Sentient Gelatinous Cube is squeezing themselves into a shape that can wear armor or clothing, they cannot use their Engulf trait. If they are wearing armor or clothing that fully covers their body with no holes or openings then they also cannot use their Pseudopod trait (Talk to your GM about the specifics of your armor or clothing).

Sentient Gelatinous Cubes can try to squeeze themselves through small openings or to other non-permanent non-standard shapes, but whatever they do they must be able to fit their brain and eyes which together are roughly 30cm by 30cm by 30cm (Or as a very rough rule of thumb the size of a loaf of bread). Their brain and eyes also cannot squeeze or change shape like the rest of their Gelatinous body and can take damage if attempted. Squeezing themselves to such extremes is not easy and cannot be done for very long, to do so for longing than 6 seconds requires a Fortitude saving throw with DC 20, and each successive 6 seconds requires another Fortitude saving throw with a DC of +10 to the previous Fortitude saving throw DC (First time DC 20, second DC 30, third DC 40, etc).

The Sentient Gelatinous Cube can move into a Medium or smaller creatures' space, engulfing and beginning to digest the creature or creatures. Whenever the Sentient Gelatinous Cube enters a creature's space, the Cube must make a Constitution check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use).

On a successful save, the creature can choose to be pushed 5 feet back or to the side of the Cube. A creature that chooses not to be moved suffers from the results of an unsuccessful save.

On a failed save, the Cube enters the creature's space and the creature takes 3d6 acid damage and is engulfed. The engulfed creature cannot breathe, is restrained, and takes 6d6 acid damage at the start of each of the Cube's turns. When the Cube moves the engulfed creature moves with it. If a creature dies while engulfed by the Cube, the Cube can spend a bonus action to choose to fully consume the organic parts of the creature leaving behind only its items and gear which the Cube can then freely choose to spew out or the Cube can freely spew the entire corpse out. Fully consuming the organic parts of a creature that has a challenge rating above 0 gives the Cube inspiration for 8 hours or until the Cube takes a short rest.

An engulfed creature can attempt to escape the Cube by taking an action and making a contested Check (The Cube makes a Constitution check, while the creature making the attempt makes either a Strength (Athletics) or Dexterity (Acrobatics) check based on their choice). On a success the creature can choose to exit the Cube 5 feet away in any direction.

A Cube can only engulf one medium creature or four small creatures at a time.

The cube takes up its entire space. Other creatures can enter the space, but a creature that does so is subjected to the cube's Engulf and has disadvantage on the saving throw. Creatures inside the cube can be seen but have total cover. A creature within 5 feet of the cube can take an action to pull a creature out of the cube. Doing so requires a successful contested check (The Cube makes a Constitution check, while the creature making the attempt makes either a Strength (Athletics) or Dexterity (Acrobatics) check based on their choice), and the creature making the attempt takes 3d6 acid damage. 

If a Sentient Gelatinous Cube is wearing armor or clothing, they cannot use their Engulf trait.

The Sentient Gelatinous Cube can extend parts of itself away from its Cube body to interact with its surroundings. These Pseudopods can be used in place of hands, but they lack the intricate articulation of fingers, instead gripping things through a sticky slime excreted onto the end of the Pseudopod. The grip of the Pseudopod is strong enough to hold and use an object (such as a sword), or to interact with an object (such as using a doorknob). No more than two Pseudopods can be formed at any given time.

As a Natural Weapon attack a Pseudopod can be used to hit a target, using its Strength modifier, doing 3d6 acid damage.

If the Cube is wearing armor or clothing that fully covers their body with no holes or openings then they cannot use their Pseudopod trait (Talk to your GM about the specifics of your armor or clothing).