Comment 1 for bug 1638063

If you're using Rhythmbox 3.x (which I assume you will be if you're using a current version of Arch), only the Gstreamer 1.x architecture will work. You cannot use plugins from Gstreamer 0.10.x in Gstreamer 1.x.

If the gst-plugins-bad-1.0 package in Arch does not include the soundtouch plugin, then you will have to first obtain libsoundtouch (either from Arch packages or compile it if it isn't in the repo), then recompile gst-plugins-bad-1.0 and make sure that it detects libsoundtouch in your pkg-config directory.

Unfortunately this is not a bug that I can fix from my end because I am not responsible for what packages Arch Linux does or does not include in their package repository.