Hi Steve,
I'm confused, are you wanting to export the entire mix or individual tracks? you said only the last track is heard in the created wave file, so that would lead me to believe you're wanting the ENTIRE MIX exported, not individual wave files.
if you select TRACKS as your output source, each of the tracks you select in that window will export as a SEPARATE wave file, which is what I thought you were wanting to do, thus my advice above. but if you're wanting to hear everything, then select ENTIRE MIX as the output instead.