Rip stereo to mono

Any decent audio editing program should be able to do that. Although I am not specifically familiar with it, I suggest that you check out the free program Audacity. Its latest version provides FLAC support, and it is well regarded. It is available for Windows, Mac OS X, and Linux.

I use a Windows-only professional program called Sony Sound Forge Pro, which is expensive, but I would expect its less expensive consumer-grade counterpart, Sound Forge Audio Studio (shown at the same link) to be suitable as well.

Regards,
-- Al

Wouldn't it be easier to make the rooms mono?

Al is right about Audacity, it's fairly easy to convert stereo tracks to mono using it, though I haven't tried it with FLAC files. Instructions are here.

It still easier to convert the stereo signal to mono using Y cables.