After testing CorePlayer and some others, I have to say that I moved back to TCPMP again. CorePlayer is great, and it´s greatly optimized for handheld devices, but lacks AC3 audio support.
TCPMP on the other hand, although it has been deprecated, perfectly supports AC3 and many other formats, and is still the best player for WM devices.
There is a recomp version specifically compiled for Windows Mobile 6.1 that works flawlessly, and includes most of the plugins available.
You can download from here:
http://www.htcmania.com/showthread.php?t=51241
Don´t worry, it´s free.