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.

The question:

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...?

Hey, I saw your post in my nexus thread for my "no-disembodied voices" patch; just have not had time to respond yet, Sorry. I honestly don't know the answer and I am not really all that experienced in converting files from stereo to mono. Hopefully someone more knowledgeable can provide some guidance. I would need to get permission from Cliffwoms to upload any modified sound files, since my permission was only for uploading a batch of renamed silent files, so I would want to ask him before going beyond that to general bugfixing (assuming it is a "bug" in the first place).

Try the http://manual.audacityteam.org/man/edit_chains.html- select Stereo to Mono. How to do it is http://manual.audacityteam.org/man/chains_for_batch_processing_and_effects_automation.html-.