sound: Fix swapped parameters for function.

CheckVolumeSeparation() has the sep and vol parameters backwards
compared to how they are used when the function is called. Reverse
the order to fix this (thanks proteal). Fixes #330.
Merge pull request #1 from chocolate-doom/master

sound: Fix swapped parameters for function.
Merge pull request #2 from Gio/master

Merging bugfixes from chocolate-doom/master