* ReplayGain is metadata contained in the FLAC file that indicates whether and how the volume should be adjusted during playback. No audio data is changed--this is info for the decoding software only, and can be enabled/disabled by the user. Decompressing a FLAC file into a WAV file yields the original WAV file, bit for bit.

* FLAC compression levels differ in the complexity of the algorithm used to compress the original audio. At higher levels the compression process is more demanding on your hardware and takes longer to complete than a simpler, lower compression scheme, but the resulting file size is normally smaller. Audio quality is the same regardless of the compression level. I suppose higher levels create more overhead during decoding, but I haven't noticed any differences in seeking or such.