Problem using LibAo on Windows 7

I'm trying to use LibAo on Windows 7. The ao_default_driver_id function returns -1 (it can't find any default sound output). I really can't figure out what开发者_如何转开发 I'm doing wrong here. Does LibAo require something more to work?

ao_initialize();
int driver_id = ao_default_driver_id();

/Thanks in advance!

I'm using libao as well for a project. To help you troubleshoot, did you build the library yourself or did you use a precompiled binary?