Using the mp3-player "mpg123" version 0.59r compiled WITHOUT alsa support (use OSS only), but the system's soundcard running with alsa-driver 1.0.2 didn't have any audible sound output, although:
1) 'aplay' plays a WAV file without problems.
2) alsa-oss modules loaded and /dev/dsp exists and 'works' (tested by cat-ing WAV to device)
...so I've tried using mpg123 version pre-0.59s which also would NOT compile with 'linux-alsa', so I also compiled with standard OSS support.
This worked. (maybe this information helps someone!)