It was not a stupid question. I tried Googling the phrase "post free MP3s", and it was not obvious from the Google listings that any of the first 10 sites would accept contributed mp3s in the same way that, for example, Flickr accepts contributed images.

Unfortunately, the web is not set up in such a way that there's an easy way to find the answer to the question posed by Sadie from Shady Grove. Finding the answer has much more to do with web searching skills than with musical ones, and perhaps Sadie's expertise lies elsewhere. One thing that Sadie needs (although she didn't say so) is some advice about useful sites, in the experience of the sites' users. Searching the web to find out what users think about various sites is yet another uncommon skill. Finally, I presume that Sadie wants a site where the mp3s she posts might actually reach people who care about what she's recorded, and there's a wide range of vocabulary ("folk", "traditional", "acoustic", etc.) that might be relevant to that. Choosing the wrong vocabulary would limit a web search inappropriately.

I think Sadie made the right choice to seek community advice on this one rather than striking out on her own, armed with nothing more powerful than a search engine.