First of all let me clarify for the record that I am not a lawyer, and that the legal reference for such questions is on http://dev.live.com/terms/ and various sub pages.
Now, as an indication of what this means, my understanding of the terms of use in plain English
is that regardless of the content you store in Silverlight Streaming -- Silverlight applications, media assets (audio, video, images) -- the metered variable that will matter when the service is no longer free is the aggregated bandwith usage, which should not be over 5 TB per month per SLS user account.
So if you only stream audio content from SLS, the monthly bandwidth usage generated by your web site should be below 5 TB per month for the service to remain free. Makes sense?
Cheers.