The volume level isn't specific to any file type; it comes from the source. There is a feature called ReplayGain that can adjust the volume level of a group of tracks to the same level, either by applying a permanent volume change to the files, or by including a meta tag in the file that tells a compatible DAP how to adjust each file on the fly, without modifying the original recording. Some of us are petitioning FiiO to include support for this tag in a future firmware update; it's listed on their Bugzilla page as an enhancement request.