If your soundcard is not supported, several USB-based audio system are supported by NetBSD, to work around this.
Some older notebook models don't support output at full CD quality (44.1kHz, 16bit stereo). Software needs to account for this on such systems, e.g. for mpg123 either the "-4" switch has to be used, or "-2 -m" will help, too.
NetBSD provides an audio API that is closely modeled after that of Sun's Solaris. For compatibility with other systems, there's also a Open Sound System (OSS) library available in the NetBSD base distribution.
Please see the download page for some optimized kernels & kernel config files, and also have a look at the general NetBSD documentation.