Answer by qwertymodo for Change device controlled by volume keys without changing default

BLARG's answer worked for me, but I decided to see if I could get it to work with just AutoHotKey, and it turns out that yes, you can.

; Use the Soundcard Analysis script found here to set these parameters; https://www.autohotkey.com/docs/commands/SoundSet.htm#SoundcardDeviceNumber := 7ComponentID := MASTERVolume_Up::SoundSet, +2, %ComponentID%, VOLUME, DeviceNumberreturnVolume_Down::SoundSet, -2, %ComponentID%, VOLUME, DeviceNumberreturnVolume_Mute::SoundSet, -1, %ComponentID%, MUTE, DeviceNumberreturn

Once you've set the device parameters correctly and confirmed that it works, you can compile the script into a .exe using the AHK-to-EXE converter that comes with the AHK installation and link it in your startup folder to automatically start it at login. If you want the volume to increase/decrease more quickly, you can do so by modifying the +2/-2 value (I believe 2% is the default change per press on Windows, but the rollover may be slower than without AHK).