Aren't the original Amiga music in MOD format which NeoLemmix already supports (and also, aren't those basically what NeoLemmix already comes with)?
Don't get me wrong, it's fine to have OGG conversions for Amiga for many other purposes, just that for inclusion in levels, it'd be less to download for levels to just reference what NeoLemmix already has when they need Amiga music.
Even for other Amiga Lemmings games that NeoLemmix doesn't include music for, if the MOD or MOD-like format used is supported by NeoLemmix, it'd be smaller files for levels to use MOD format compared to OGGs. OGG also technically is lossy compression and so isn't going to be a 100% perfect match to original, even if differences probably unnoticeable in most cases for most ears.