Yes gigapus, "mplayer -ao alsa file.wav" works fine.
And, steffen800, i found out that play-sound

(espeak -v de 'Hallo Welt' -w /media/mmc1/speak.wav && play-sound /media/mmc1/speak.wav)

also works.

But you have to set the system sounds to 1 or 2.
It seems that "play-sound" is linked to N800´s system sounds settings.

Does anybody know whether there exist any parameters for play-sound?
Or does play-sound only plays wave files?

Markus