I found such warnings using Message Logger http://www.nexusmods.com/oblivion/mods/45870/?
2017/01/04 22:14:25 [006AAF4F] [WARNING] Attempting to play 2 channel sound "fx\cliffworms\soundsofcyrodiil\miscsounds\SoCbeenest.wav" in 3D (must be mono)
2017/01/06 21:05:53 [006AAF4F] [WARNING] Attempting to play 2 channel sound "data\sound\fx\ambient\regions\birds\forest\SoCFlyBy1.wav" in 3D (must be mono)
2017/01/06 21:07:43 [006AAF4F] [WARNING] Attempting to play 2 channel sound "data\sound\fx\ambient\regions\birds\forest\SoCFlyBy3.wav" in 3D (must be mono)
This is from the mod Sounds of Cyrodiil. Both http://www.nexusmods.com/oblivion/mods/39804/?and http://www.nexusmods.com/oblivion/mods/46685/? versions.
There are more stereo sounds there probably.
will stereo wav files be played by game engine or not? Or will they be only played in mono (one channel?)...
If they won't be played at all, I read that Audacity program may be used to convert them to mono. Also, is there any good way to tell that a file is in stereo efficiently, because there are MANY files to check out...?