Implement ReplayGain / Sound Check functionality on track ingest
When tracks are ingested into the library (or dragged to a player) a "loudness" analysis should be performed and the tracks should (perhaps optionally) be replayed so that they have equal subjective loudness.
Any signal processing should be applied to this nominal loudness (which would avoid excessive AGC or pumping).
The overall target level might be adjustable but -14dB relative to full scale seems a reasonable default for most "pop" music applications.