It could be useful to be able to hide an app from EarTrumpet just because I don't use it (like "System Sounds") in order not to cover too much screen and be focused on what I work with.
Yes, please. Also, just generally having each audio source take up less horizontal space on the mixer would be very useful. I often have the mixer open and extended to display every audio source and it takes up a very large amount of real-estate. I'd like to see the sliders shrunk by half, and the ability to hide the system name for each device (just show the user-defined name). This would make the mixer a lot more useable for me.
Adding on to this, maybe even the ability to "pin" certain apps so they show above others.
+1 to pinning apps/devices and hiding them.
In case this is an XYproblem I figured I should explain why.

System Sounds and Discord are currently listed for EVERY device, no matter if they are making noise or not, or whether they are being played to a specific sound device configured inside the program.
Generally speaking, I don't ever need to adjust these on devices they are not outputting on, so they just add to the clutter.
Ideally System Sounds and Discord would only show up on the devices they are actually using. But I'm guessing this is a 3rd party bug due to keeping a sound session open for some reason or another.
I have a lot of audio inputs/outputs that are needed frequently enough that I can't just disable them, but I never actually run programs to them (they're hardware inputs that I need for recording) so being able to just hide them from EarTrumpet would be amazing.
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Adding on to this, maybe even the ability to "pin" certain apps so they show above others.