Can't Split Tracks at Playhead for Imported Mixdown in MultiTrack

Not quite sure what you mean by 'split the track at the playhead'... but if you want to split a multichannel file of anything in Multitrack, then you can use the twirl-down next to the track name which will reveal each channel separately, allowing you to insert them onto different tracks if you want. So with a stereo MP3 you'll get a Left and Right channel shown that you can insert independantly of each other.