Created attachment 707133 [details, diff] libsdl2.patch SDL2 supports sndio by default, but the support has to be enabled via a configure flag. The attached patch allows enabling sndio support through an USE flag.
Fixed with commit 8fc630eb6356c8c2a9ebafdb5ee7a1d6dba2cdf7 https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=8fc630eb6356c8c2a9ebafdb5ee7a1d6dba2cdf7