If you have your own place, you probably get an Electrical bill every month. The amount of the bill is determined by how much electricity used (measured in units) times how much each unit cost. Data Transfer is the same thing and each Hosting account usually comes with "X" amount of Data Transfer per month, to start with. Go over that and you pay "X" amount per unit -- usually measured in MB or GB.
Now imagine if you were only allowed so many units of electricity, it's winter time, and you've gone over you monthly limit. If you couldn't pay for the extra units of electricity, it would be cut off. Same with Data Transfer. So the more you figure you need, the more you should included with Hosting account.
Add up your sound files (in MB) and mulitply by 100, then 500. That should give you an idea of low/high Data Transfer you will need each month. Mind you, if you site gets popular and the downloads run into the thousands, you will need to make sure your Hosting account allows for upgrading your account.
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