yeah you can set individual price tag for each downloadable file, so upload multiple versions of your game and set a different price for each
For example if you want to offer the sound track along with the base game for an extra $3, set the base game price to $4 and the soundtrack price to $7, so when someone pays $7 they get both the soundtrack and the base game.
Now if you want to offer these DLC separately (for example both DLCs are $2 but you want it so players have to choose which DLC they get from paying just $2), then it is not possible in Itch.io in the same game page.
What you can do in this case, is to create a new game page for each DLC of your game (make sure these pages are unlisted so it doesn’t spam the itch.io search index), and in the main game page, you add the links to your DLC pages so players can go over there and purchase them separately.