Music cards that play lower/slower songs as battery fades

Musical cards and other battery-operated musical flotsam that show up this time of year start sounding pretty sad as the battery wears out.  We have this annoying musical door mat that plays ‘Up on the Housetop’ when stepped on, but takes some obscure-sized watch battery that I’m not going to invest in anytime soon and so it’s sounding really sad now.  What it needs is a new song with a naturally lower key pitch and slower pace.  An infrequent sampling of the battery voltage should be good enough to tell the processor what to do next.  I’m thinking it starts with ‘Carol of the Bells,’ then transitions to ‘Silent Night,’ then ‘Auld Lang Syne,’ and by Good Friday, it plays ‘Even the Heavens are Weeping,’ a horrid dirge of a hymn only sung on Maundy Thursday or Good Friday.  Near the end of battery life it could do the ‘Volga Boat Song’ or "Daisy."