I would asume at the top of your battle handlar script and at the end of it.
I'll give it a test before I can say for sure.
$game_system.memorize_bgm
$game_system.resume_bgm
...
:O_o:
The methods may not be named the same but look in Game_System to find them.
these methods are not the same for when you even call the script $game_system.memorize_bgm it crashes so to say
$game_system.bgm_memorize
$game_system.bgm_restore
are the ones that works for me >.> but they DO NOT remember the exact point in the song it was remembered JUST the song that we being played
*edited at 11:18 pm*
Oh I remember something the BGS and the ME do what you want a bit
the bgs could be playing then you play an me, and the background contiunes where it left off when the me finished. Perhaps you can inport your battle music into your ME resorce and then ...work from there?