Another try to resolve sk8trix's problem.
Please don't take offense if you already know what I'm about to say next, keep in mind that I have no idea about your proficiency when it comes to software; you might be a genius, and you may be just an average joe like I am, so I'm taking a shot in the dark here...
Perhaps you have disabled a service required by the phone's default music app ? The software I listed in my prior post are apps, but there's a distinction between apps and system services. You may have stopped an essential software service either voluntarily or involuntarily and caused this. Here's what running on my phone, both app and services, with the music player working as expected:
Amongst the apps I have working at the moment:
Epinephrine
Market
Opera mini
Files
Music
MediaShare
Browser message
No other app shows as running in task manager... From the "Manage application" menu in the "System" applet, under "Running Services", Here's what I have:
ExtDispService
WifiApNetMonitorService
AtCommandService
SyncEngineService
ScheduleService
CacheService
WeatherService
WeatherWidgetProvider
GPSToggle$UpdateService
MediaSyncService
GlobalUnplugService
GoogleMessagingService
GfxService
PersonalPortalService
DownloadServer
DataManagerService
Multi-Touch Keyboard
Also My battery settings are set to "Maximum battery saver". Perhaps you have made a custom setting which would throw things off ?
I believe that if you match my own settings, it should work. This said, I may have missed something. Also, Perhaps we run a different version and everything works in a slightly different way. So, try it and see, but just be aware that your mileage may vary.
Android version: 2.2.1
Kernel version: 2.6.32.9
Build #: OLYLA_U4_0.37.4
Good luck.