That's a great question. I don't think that's actually something a program has a choice over -- the OS typically deals with mixing multiple audio streams into a single output. I'll keep in mind to see if there's a way to force exclusive output, but I'm confident that there isn't a portable way to do it -- certainly not with the PortAudio portable audio library I'm currently using. I know Linux and Windows both have relatively simple interfaces for the user to mute other programs and system sounds. You can also programs to play to specific audio devices (not necessarily the default), so a second sound card would again be a way to ensure that isn't a problem.