Short answer, you probably can't right now..
Originally Posted by BeMacized
I wanted to try connecting my Razer Kraken 7.1 headset to my shield via USB OTG, and to my surprise, it worked. When changing volume though, it only seems to have 2 modes: no sound at all, or louder than my desktop speakers can possibly go. I was wondering whether it would be possible for me to change my volume somehow, so that i can actually wear my headset while playing music through it. Is there some kind of solution for this?
Thanks a lot!
Seems like a Android limitation.
So I've been looking around and I found a thread here for the Nexus 7 (2012) trying to sort out this exact issue. Nexus 7 USB Audio Support
(and a little more)
Basically jacknorris created some custom kernels that support USB audio out for the Nexus 7, and a mod that allows for volume control.
He also made a Flashable Zip that adds just the feature you're looking for, but it's of course for the Nexus 7. Flashing it would probably mess things up, like no audio.
You could try messaging him and asking nicely if he would help you compile a fix for the Shield, he seems to have an idea of what to do with the N7, and the Shield is pretty darn stock. It's a possibility you could supply him with the files he needs to mod and then you could test them for him.
It looks like that thread is still active though, so you could try asking there first.
It's either this, or go and file a bug report from the shield.
Even though it's really something that Google should fix.
Settings > About Shield > Send Feedback to NVIDIA
I just sent feedback under Software Enhancement for this bug.