I second TheSword's recommendation. The 6.50/month subscription gives you access to everything.
I also have an Apple Music subscription. I'm finding that I use it more than Syrinscape as thematic background music is mostly what I want and it is less fiddly to select a song or playlist. Generally, I play from Apple music and use Syrinscape for specific encounters or areas.
For Syrinscape I used a playlist someone already made for Curse of Strahd and shared. In addition to the items that TheSword mentions, there are also some witch battle soundscapes and a variety of other horror and suspense themed pieces. These include:
Seedy Tavern
Witchwood
kc Ravounel Forest
Storm
Mountain Pass
Dungeon Depths
Icy Wasteland
Disjunction Chamber
Spooky
Undead Battle
Zombie Battle
Yeth Hound Battle
isos Prison of the Mind
isos East Ward
isos Dead Don't Dream
isos Refuge of Dreamers
I have two playlists I use in Apple Music: a "Vistani" playlist and a general "Curse of Strahd" play list.
For my "Vistani" playlist (e.g. to play in background when the party visits a Vistani camp):
Gypsy Caravan: Russian and Tzygame Folk Guitar Music, Andrei Krylov
Music from Romania, Various Artists
For my general "Curse of Strahd" playlist (for general background music), I selected from the following albums:
Halloween Black Moon: Dark Ambient Music, by Halloween Music Specialists
Ambient Music for The Twilight Zone, Soundtrack 2011
Cold Hands, by Horror Music Orchestra
Light Piano Horror Ambiance, by Bobby Cole
I selected 74 songs from these albums, which make up just over 5 hours of background music.