I created a virtual machine with aqemu and the sound was working properly in the vm. Then under Media -> Computer Ports -> Add USB Port I added a passthrough for my mixer connected via USB. After starting the virtual machine again the sound was distorted to an almost unrecognizable state (maybe I should upload an audio file of it). It sounds very "digital" - like old game music. When I remove the passthrough again the sound is back to normal. The USB passthrough works as I can use the controller with the vm. (It's a different distortion than what I described here.) The headphones are connected to the USB mixer controller which has an internal soundcard. The sound is normal when I connect the headphones to my PC instead of the controller.
What might cause this and how to prevent this? The sound worked earlier but I'm not sure whether it was with QEMU/KVM or only in VirtualBox.