Openmoko-mediaplayer2 is a program for play MP3. Sice this media player currently is based on the pulseaudio sound server --- and this sound server breaks all phone applications using sound as soon as the phone goes into power saving mode it is unwise to install this application at this time.
If you have already install this application you have to uninstall pulseaudio, as well, to get your phone working as a phone again.
This Media Player is basically useless due to the default theme, to. To install with a better theme run:
opkg install openmoko-mediaplayer2 wget http://abraxa.dyndns.org:81/random/openmoko-mediaplayer-theme.tar.bz2 tar xjf openmoko-mediaplayer-theme.tar.bz2 -C /usr/share/themes/Moko/gtk-2.0 rm openmoko-mediaplayer-theme.tar.bz2