Playing audio files from the command line is now fine. But I still have the following problem with gmplauncher (I don't know if it is a known bug): once I start to play an ogg file (on the internal memory card), gmplauncher doesn't react until the end of the ogg file. When gmplauncher is run from a shell, I get the following message only:

gmplauncher[10199]: GLIB DEBUG ConIc - con_ic_connection_send_event(0x8b938, Vinc17, WLAN_INFRA, 0)