Go to this
link and download the plugin .exe files to your PCs Desktop. Cradle your device for an Activesync connection then open the .exe files on the PC. Activesync will do the rest. You'll want the AC3, FFMPEG, SPEX and VORBISLQ plugins.
An additional plugin not on that page is for Non-DRM AAC files. You can get that one, the BetaPlayer AAC plugin 2005-08-02,
here. This one is a zipped .cab file. After you unzip it transfer the aac.arm.cab file to your X5s My Documents folder an tap on it.
HTH