Eternum Music Restoration Mod
TL;DR of what's below: Download mod > grab game folder > put in eternum folder. Unless you've unpacked the RPA you won't have to overwrite anything
Ok it seems that the game prioritizes /game/audio/music over the .rpa version, so all that needs to be done is unpack the zip, grab the game folder and drag it into your Eternum folder. That should restore the music back to the original songs. I've tested it on a few songs in a freshly unpacked 0.8.6 version (windows, specifically) and it played the original music.
Unless Caribdis has to remove more songs again in the future, this
should be futureproof and safe to use in all versions from 0.8.6 and above to restore the music that was used in 0.8.5 and before
EDIT Feb 12:
I've combined Pilgrim1 and Pilgrim2 and renamed it to the new name (lunasex1)
I've also added riofluye and riofluyeb under their new names
This way it won't require any script changes at the price of the mod being perhaps 8 or so MB larger, and also should make it fully compatible with LightmanP's multi mod whether he changes the scripts to reflect the new updated names or not