Is there a way to get Audacity to “Batch Normalise” them all to the same level.
Note that normalizing won’t make them sound equally loud.
If you want them to sound equally loud, try [u]MP3Gain[/u] or use a player that supports [u]ReplayGain[/u], or if you have Apple hardware or ITunes, switch-on Sound Check.
MP3Gain and ReplayGain can batch process all of your files, or there is an AlbumGain mode where one album/folder is processed at a time to volume-match the level of the album as a whole while maintaining the relative loudness difference between quiet and loud songs on an album,
Note that all of these loudness matching techniques will make many/most songs quieter. That’s because many quiet-sounding songs are already normalized (maximized), so the only way to match volumes is is by setting a lower target level, making the loudest songs quieter.