Get CDMage, open the image and see what it contains. If it came with mp3s it probably doesn't contain audio tracks. And do you have any special reasons for converting it to a mode1/2048 image? (CDMage can do that if you really want to.)
By the way, the Tech Help forum is better for these kinds of questions, this hasn't really got anything to do with the SegaCD.
If its a SegaCD game i doubt it's a mode2 track, SegaCD games are mode1. If its a saturn game then there are games where audio (and video) data is in the mode 2 track but this is not the cdda audio. And you cant convert a mode2 track from saturn discs to mode1/2048 (ok, cdmage will do it but you will screw up the data). The mode 2 sectors here are "real" mode2 sectors - 2336 bytes/sector in cooked form - and not the usually as mode2 refered cdrom/xa mode2 form1 (2048 bytes/sector cooked) or form2 (2324 bytes/sector cooked) sectors. Only thing you can do is to save them as mode2/2336, but why ? To save ~2mb on game like NiGHTS with a 175mb (raw) mode2 track? There are some games like Winter Heat where the cdda tracks are as files on the cd too, if you load the image of the first tack (mode1, ~50mb) into cdmage it will show that there are more than 350mb data. The files here are actually "pointers" in the iso refering to the cdda tracks beyond the first (2) track(s) and so you need the mp3 or wave files too.
AFAIK, there are no Sega CD game that has a MODE2/2352 track. You must be looking at a Saturn game or *shrugs* a PSX game. Did you find out the MODE2/2352 info from a cue sheet by any chance? If so, then that cue sheet would be the file to open in CDmage. Before you go and convert your image, you might want to try burning it with BlindWrite if CDRWin or Nero doesn't work with your burner (load cue sheet, select speed, and burn).