When I create a playlist with songs by several different bands, some songs are louder than others even though the volume of my system remains the same. How can I fix this?
Recordings are all mastered at different levels. Depending on the producers preference the final mix for an album will be louder or softer relative to other albums, although generally all the tracks on any given album will have a common threshold for loud and soft across tracks. The fix for this will vary slightly, depending on which media player you are using.
In Windows Media Player click the menu drop down and choose View > Enhancements > Crossfading and Auto Volume Leveling. Click the Turn on Auto Volume Leveling link. This allows Windows Media Player to adjust to volume differences between song on-the-fly. From this enhancement you can also turn on cross-fading which offers a more professional sounding transition for fading between tracks.
For iTunes users, the wording is slightly different but the result is the same. Open the iTunes Preferences from the Edit menu. Click the Audio tab. Check the box next to Sound Check to have iTunes automatically manage volume adjustment for your music.