Samuel Wood (Otto) on "oembed Soundcloud sets fixed width instead of 100%"

But you shouldn't need to specify it as 100%, that's their default value.

Look, their docs say the maxwidth defaults to 100% here:
http://developers.soundcloud.com/docs/oembed

And if you actually test the oembed URL without specifying a maxwidth, sure enough, the resulting iframe says width=100%:
http://soundcloud.com/oembed?format=json&url=http://soundcloud.com/forss/flickermood

Note that oembed only processes *on post save*. If you're just adding this code and not actually updating your post or making a new post, you won't see any changes.