Trouble with playback of a multi-track recording

[fab five]

Hi, newbie here. Windows XP. Please pardon the fact that I don't use correct terminology here as I'm just using Audacity for personal purposes, not professional music/sound studio here.

I am trying to make a recording that has 15 little tracks on it of people talking (taken from old cassette tapes), with 5 different background songs playing (one song after the other). The whole recording is about 12 minutes total (each little track of voice is about 30 seconds to a few minutes long)...

I have successfully put the recording together, saving and running playback here and there. But when I finish and try to play back the entire thing, I get lots of muffled, scratchy noises sometimes and the background song plays twice like an echo. Does this have to do with it being a stereo track. Can I not merge stereo and mono tracks together into one big recording? (you know, like having sound effects behind your voice, only it's a song?) How come it sounds great as I'm working but when I finish, it sounds terrible??? Is it a computer memory issue? (I have no idea how much memory we have, we're XP and it's been a great computer for us but now?)
I have made this recording 3 times with the same results each time. Wanted to give this to my husband for a gift but darn it anyway, it's not working !! (the cassette tapes are of his grandpa talking to him when he was a little boy) Thanks for any help/thoughts you may have!