If you use VirtualBox to run things (like Windows) under Linux you know that its pretty capable. It can even map sound from the guest OS to the Linux ALSA system. However, the GUI only provides a way to use the same input and output device on the host OS. That’s a problem for me. I have a normal PCI or onboard sound card (I have both) that I use for output. But for input I use a USB microphone that has no output.

It turns out there is an environment variable for this. If you set:

VBOX_ALSA_ADC_DEV

you can select a different sound device for input. For example, my microphone is set to hw:3,0 (getting a USB device to come up as the same card every time is another post for another day). So before launching any VirtualBox I make sure to have this environment variable set:

VBOX_ALSA_ADC_DEV=hw:3,0

Works great!