Ringtones incompatible with custom rom. How to improve compatibility?
It's weird that your ringtones are still incompatible after conversion to the Ogg Vorbis format, because it (as you probably already know) is what Android's native ringtones come in.

Since you mentioned that you put your files in /system/media/audio/ringtones, it is worth a try to check and set the permissions of your files. It's as easy as


  chmod 0644 /system/media/audio/ringtones/*


in a terminal emulator or through ADB.

If you want, you can also put your ringtone files in /sdcard/media/ringtones and do a reboot. You should be able to see, preview, and set your ringtones in Settings. (This works with regular MP3 files, as I've tested months before.)

P.S. Since you also said you've tried using Audacity, it's worth noting that Android doesn't repeat custom ringtones when they stop playing (as is the expected behaviour with call ringtones) unless you include a tag named ANDROID_LOOP set to true in the ringtone's metadata.