Main article: Left 4 Dead
We can't look at the source code to check, we must rely on developer commentary etc.
The presence of music files doesn't prevent the music from being procedural, the Music Director might sample from that music and stitch it into something new.
Here are relevent quotes from the developer commentary (I think all the music commentary is on the hospital section of the commentary level, I haven't checked the other parts).
Tim Larkin: We took several steps to keep the music interesting enough that the players would be inclined to keep it on as they play. We keep it changing so it won't become tedious; to this end, we created a music director that runs alongside the AI director, tracking the player's experience rather than their emotional state. We keep the music appropriate to each player's situation and highly personalized. The music engine in Left 4 Dead has a complete client-side, multi-track system per player that is completely unique to that player and can even be monitored by the spectators. Since some of the fun of Left 4 Dead is watching your friends when you're dead, we thought it was important to hear their personal soundtrack as well. This feature is unique to Left 4 Dead.
Mike Morasky: A lot of over-reaching dynamic music systems go to great lengths to organize and control a very expressive art form, often to the point of making the results perfectly 'controlled' but also perfectly predictable. Our Music Director aims for 'planned serendipity'. By designing the music and rule sets to increase the probability of beautifull happenstance and to minimize the probability of inappropriate mistakes, we end up with the highest percentage of musical events working as planned, an nice mid percentage of acceptably artfull mistakes, and very few actual poor moments. If you overdesign the music and rule sets, there are no surprises but without surprises, listeners are quickely bored. Ironically by keeping things simple, the music seems planned; greater complexity just leads to greater randomness and many more poor moments.
Mike Morasky: We based the music on what the player is actually experiencing and not on what we want them to experience. Working from artificial life work our designers had done on Lord of the Rings and the Matrix sequels, we implemented a simple system to examine what's going on in the player's immediate environment, the added the appropriate reactive, scalar rule sets to cantrol the music and its volume levels. Most of the more prominent musical events are thus reactive results from rule sets processing this input — making the musical experience specific to each player. This system also controls the dynamic mix system — another feature unique to Left 4 Dead.
At the very least there is a set of tracks that are mixed to give different feels as well as musical events that are triggered on "witch alerted" or similar events. Though the notes and instruments are not procedurally generated, it would be fair to say that the music is dynamically mixed and arranged.
If you have the game, the music resource files are at:
steamapps\common\left 4 dead\left4dead\sound\music
With the exception of the non-interactive parts of the game they are all pieces 5-30 seconds long that, if mixed with other pieces, give the in-game music. The longest sequences are the "you are dead", "safehouse reached" and "ending credits" music, those are always identical in-game without mixing.