Is this something that you find compelling? Does sharing sound intrinsically valuable to you? Do we need a social network just for sounds? I don't get it.
Sound can be music. SoundCloud is a very good platform for professional and amateur musicians to promote their work. The interface is slick and for the kind of music I'm interested in (reggae, dub, electronic music, drum'n'bass, dubstep...) there's a vibrant community of musicians regularly uploading great sound. I like the fact that SoundCloud does only one thing and does it very well.
music yes, but not necessarily all sounds. However, if they'd pitched it as 'music', people would say 'myspace, been there, done that'.
"Sounds" are one of those things that have to be consumed synchronously and as a whole (generally speaking). Long and short form text, and pictures, can be compressed, sliced up, moved around, and we may get the gist of it - and can easily share.
Videos... not as much, but we can still take a small segment of a video/movie and get value from it.
Songs, probably similar to movies, but sounds as a broader topic, not so much. At least, not to me.
