Hi Dylan, could please you explain a little bit more ?
If I understand correctly, the use scenario is:

1. Record from default mic.
2. Add a pinned stream to record from USB (an HDMI capture )
3. See system gain changed and it affects default mic recording.

 >>> Pinned streams should only be affected by the node volume.
I don't understand this.
Do you mean the expected behavior is that the gain limit of pinned device should not affect default mic ?
But there might be other normal stream sharing the same device with pinned stream.

I agree that we don't have a clear gain behavior with multiple active inputs.
We can not determine a single system gain limit for two different devices.

Yes, if the pinned stream and the normal streams aren't sharing a device then the two shouldn't affect each others volumes. It's probably the normal use case. I doubt there is a lot of usage streams capturing from multiple devices at the same time. Link did for keyboard mic, but that's a dead feature at this point anyways.
The goal is to allow different streams on different devices to have independent gains.

dgreig@, I could imagine usages for several simultaneous input streams. Some kind of mixer (e.g. communication hub, live music) or sound test equipment. However, that I agree that's not what this bug is about.

I have experimented a bit, and the change to the capture gain happens already when a new mic is connected to the ChromeOS device, so I am not sure if we can fix this by looking at pinned streams.  

It seems a bit inconsistent though.  Sometimes it changes and sometimes it doesn't.

Even weirder, the gain is often adjusted when a peripheral is disconnected and set to a value that is suitable for that peripheral even if it is no longer connected.

E.g. recording from a mic with the max capture gain for the mic (+6db), then unplugging the hdmi capture unit (with no active stream) and the capture gain changes to 0db, even if the mic is still marked as default.