Takes a Coupla minutes for the Track to actually start 'Playing'. What you can do is accelerate the time until the 'Playback' starts. As said - unlike LockOn, for some reason it 'Hangs' for an indeterminate amount of time prior to Track Playback actually commencing.
As regards to the Externals, all the MP Tracks have the 'Client 20091013xxxxx miz.trk designation. Once you have established which track it is you're after, copy the Track to another folder and remove (by using rename function) the .trk from the back. Load it into the editor as a Mission file and toggle Externals 'On' in the Missions Options and Save.
Thereafter just add the '.trk' back (inverse of above) to the file and you'll have externals enabled in Track Playback.