During its phases and seasons in early access, Half-Life: Cross Product Multiplayer received many themes and motifs which were implemented here and there during the game's updates leading to Season 9, the current phase of the game.
Now, with the Steam Autumn Sale, you can get Volume 1 of the soundtrack at a 70% discount. It's not going to be cheaper anywhere else.
On Steam, you'll get the soundtrack in MP3 & WAV formats, and you're free to do with it as you please - remix, convert, put it on your phone... it's up to you within bounds.