It should work with any tracks that you aren't Bitstreaming.
It doesn't matter what the source file format is. If you Bitstream MP3 files Volume Leveling will not work either. By definition, Bitstreaming prevents changes to the audio file contents, and simply sends the file as is to the output device.
Note that Bitstreaming is not the same as streaming, in the context of audio output from MC, or any audio software. For example, MC could play a FLAC file to a DLNA device, converting it to MP3 before sending it to the DLNA device. MC would then be streaming the audio, but would not be Bitstreaming it. Bitstreaming means outputting the audio file untouched by any alterations, including volume changes. Or at least DSP based volume changes, such as Volume Leveling.
Hendrik, at one time there was some discussion about allowing MC's Internal Volume to alter volume for Bitstreamed audio, as it was an easy change that didn't alter the data otherwise or something. I may be misremembering that discussion. Does Internal Volume work with Bitsreamed audio? I haven't tested.