Hi,
I dont think you would need any other software. You will just have to get your head around adding product attributes! and assigning the files to each etc.
I have just set up a shop for a friends music, where people can buy his music online. Problems i experianced was, firstly, setting up the attributes and values. But read stuff on the forum, and you will soon get the hang of it.
Also, use Zip files instead of Mp3 files. If you just use mp3 files, people may have their browser set up to automaticly play mp3's. In this case it will deduct a download from their download count.
So the process is easy, once you have got your head around setting up the attributes and values. Once a customer adds a song to their cart, they can then continue to checkout to get the song and pay for it. With my shop, i took out alot of stuff that isnt relevent to a online music store. Such as address fields, and product update options etc. So iv made the process much more simple for the user.
They they confirm the order, and get took to paypal to pay. Once they have payed, it will take them back to a page on your site which says congrats etc... and gives them the option to download their tracks.
So its all possible with Zen Cart. Wouldnt say you needed anything else.