Each song is stored as a set of wave files, one or more files per track, in the song folder under the Music folder. Their names are of the form ZZ00nn. It's not advisable to use/copy these files for playback as they are internal workfiles for the machine (for undo purposes, or recordings stopped & restarted etc), so they won't necessarily line up with each other.
To get a usable wave file for each track, use the Tascam to navigate to AudioDepot and Export each track as reqd. This uses the internal workfiles to build a single wave file for each track and stores each one in the AudioDepot folder. These filenames are based on the song name. You can then use your computer to navigate to the AudioDepot folder and copy the files out, either via USB or directly from the SD card.
If you want all songs tracks mixed into a single wave file, you need to either bounce them to a new track and export that (to the AudioDepot folder), or use the mixdown/mastering modes which (confusingly!) create files in the song folder under the Music folder; their names are based on the song name so can be distinguished from the ZZ000n workfiles.