Tracks nearest the centre sound worst
There's a Radio 4 series of music documentaries on BBC Sounds podcast now, one of which retold the history of the 12in single. A vinyl mastering engineer explains how the audio quality on a gramophone record worsens as the groove nears the centre of the disc, which is why most 12in singles never bothered to use the full surface space available but left lots of empty unetched plastic near the label. It also explains why the last track on either side of an album always sounds muffled and a bit crap.