Use the plug: ALSA devices for surround sound formats, which have automatic sample format/rate conversion.
Based on feedback from the ALSA development mailing list:
http://mailman.alsa-project.org/pipermail/alsa-devel/2009-October/022267.html

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%404081