mpg123 in CentOS linux

At first, it gave error messages like:

[oss.c:172] error: Can't open default sound device!
[audio.c:627] error: failed to open audio device
[audio.c:180] error: Unable to find a working output module in this list: oss
[audio.c:529] error: Failed to open audio output module
[mpg123.c:869] error: Failed to initialize output, goodbye.

mpg123 -a ALSA favorite.mp3
[oss.c:176] error: Can't open ALSA!

mpg123 -a alsa favorite.mp3
[oss.c:176] error: Can't open alsa!

then suddenly,
mpg123 favorite.mp3
works!
don't know why.