Deafness can be usefull way to counter a charm spell or at least reduce the effectiveness by deafening  a charmed ally to any verbal commands, so if you dont have access to a 5th lvl spell like Greater restoration it can be a life saver.

It's so refreshing to find a disabling condition spell that doesn't require concentration, even if it's second level.

This is a great way to get an enemy caster to burn up their Counterspells and Dispel Magics. A lot of spells require you to "see" a target; the caster can't see anything if it is blinded. When I see someone using magic, this is my go to spell. Additionally, Blindness / Deafness  is only a level 2 spell compared to Counterspell and Dispel Magic at level 3! Nothing is more annoying then having to counter a lower level spell. Let's burn some spell slots baby!

Honestly, one of the best debuffs in the game at lower levels. I end up using this so often, and it really turns battles quickly if the enemy can’t make their save for a few turns. A blinded enemy spellcaster is as much a risk to his own side as to your party, and several rounds of combat with advantage/disadvantage make a huge difference. Only used deafness once so far, when fighting a banshee.

Next to perfect for PVP when using a sorcerer, simply use it on the other people you're facing and if they don't know the effects or if you have it, they might be grouped up, use twinned spell and extended magic, and boom. If it hits, 24 hours of blindness, use it again for deafness, and if the dice are in your favour, they're crippled to almost any sensory effect and can't sense you. I'm doing low level pvp in a few hours, and it seems like a pretty good idea to have it.

Casting word is:

Dracolic Verbacus

It would also be great to get rid of a creatures legendary resistances

Quote from ThouAverageBrit >>

. If it hits, 24 hours of blindness, use it again for deafness, and if the dice are in your favour, they're crippled to almost any sensory effect and can't sense you.

That’s.... deliciously cruel.

The spell lasts for a minute. Not 24 hours. Upcasting affects multiple creatures, not the duration.

I think they got the 24 hour duration from Extended Spell metamagic, but that would only increase the duration to 2 minutes.

granted this is a super old post but I think it's worth commenting on in case someone reads it thinking they can do what you suggested...
"You can use only one Metamagic option on a spell when you cast it, unless otherwise noted."
also
"A sorcerer can use one Metamagic option once in the casting of a spell, not the same option more than once. For instance, a sorcerer can’t quadruple the duration of a spell by spending 2 sorcery points on Extended Spell."
EDIT: in response to ThouAverageBrit's post

good way to turn evil: blind the horses carrying a cart

so does the effect last 1 minute

Except no it says foe. Allies are not foes. Why spells that can target either allow for auto fail as an option. To @fuzzydragon

Thank you I liked the spell, I’m using it a home brew I’m making

I’m surprised this isn’t concentration.

Hello

Deafened status seems a bit weak, I'm probably going to introduce a house rule that'll make spell casting attacks have disadvantage. 

I'm going to use this at a beholder

Quote from ThouAverageBrit >>

Next to perfect for PVP when using a sorcerer, simply use it on the other people you're facing and if they don't know the effects or if you have it, they might be grouped up, use twinned spell and extended magic, and boom. If it hits, 24 hours of blindness, use it again for deafness, and if the dice are in your favour, they're crippled to almost any sensory effect and can't sense you. I'm doing low level pvp in a few hours, and it seems like a pretty good idea to have it.

Extended Spell only doubles the duration to 2 minutes.  I mean, either way, it's long enough for a combat, but it's a far cry from 24 hours.  24 hours is the MAXIMUM it can double to.  For instance, if you had a spell that already lasted 24 hours, Extended Spell would not help.