On 1.18 the music of the majority of the nether was adjusted to have more tracks play on certain biomes and to change the volume, however soul sand valley didn't change.

This happens because when the sound event name was changed from music.game.soulsand_valley (found in vanilla 1.16 resource pack sound_definitions.json) to music.game.soul_sand_valley (in 1.18 vanilla resource pack sound_definitions.json), the name was not changed on the music.definitions.json too, resulting in the 1.16 sound event playing instead of the newer one.

Both sound events can be played using the music command.