Clairvoyance

Do you need both a Glass Eye AND a Jeweled Horn to enable both seeing and hearing for Clairvoyance?

If you only have a Glass Eye, does that mean you cannot choose hearing when casting Clairvoyance?

(A literal interpretation of the text is that you only need a Glass Eye OR Jeweled Horn, and either one enables seeing AND hearing with Clairvoyance. I hope that's the intent!)

* - (a focus worth 100+ GP, either a jeweled horn for hearing or a glass eye for seeing)

Approaching this as a developer, I would parse out the condition logic as this:

either (a jeweled horn for seeing) or (a glass eye for seeing) 

Mechanically it seems to work as you desire. You only need a focus when you CAST the spell (PHB Spells section under components "A Material component is a particular material used in a spell’s casting, as specified in parentheses in the Components entry."). So you could, for example, cast it as a Seeing Clairvoyance with the glass eye, then as a bonus action change it to Hearing per the spell description. Intended or not the mechanics seem pretty clear.

Flavor you likely need both

Designer intent is probably both but without a statement it's hard to know.

Basically ask your DM. I think they should have made the focus a set that costs 100gp or 200 if hey really thought that cost matters at level 5. The real limitation on this is the 10 min casting time coupled with the 10 minute duration.  Hard to eaves drop on anything longer then 10 minutes without 2 casters or without missing big chunks of information. 

With its length 10 minute casting time, 100gp material, and poor 10 minute duration, if you're pressed under a time limit, Find Familiar is almost universally better in terms of time, money, and utility.

Quote from Aether_Veilborne >>

With its length 10 minute casting time, 100gp material, and poor 10 minute duration, if you're pressed under a time limit, Find Familiar is almost universally better in terms of time, money, and utility.

It's worth pointing out that this spell's material component isn't consumed, so if you cast Find Familiar more than ten times, you're spending more on that spell than on this one.

Also note that this spell's casting time is much shorter than Find Familiar's, so that's not really a point in the latter's favor. 

While true if you have to cast Find Familiar more than 10 times, you will get your 100gp investment's worth.

Find Familiar is both a 1st Level Spell and a Ritual Spell, meaning it can be acquired significantly earlier in a character's adventuring career and can still be cast without consuming a Spell Slot if cast as a Ritual. It also has unlimited duration once cast, meaning depending on how careful the summoner is, the familiar could last through multiple game sessions.

Then you have the Pact of the Chain Warlocks who can conjure the Familiar both without a Spell Slot and the required Hour, as a simple Magic Action. (Still costs 10gp each time though).

However the single best use that Clairvoyance has above Find Familiar, no matter how you use it, is the ability to peek inside a completely sealed room...IF you can somehow acquire the 10 minutes to safely cast it.