no problem! :)

I understand , well thank you very much for clarifying my doubts and responding.

for the saferoom music? l4d2 is unfortunately very strict on custom sounds/music, and won't let you add any custom file that's longer or shorter.
you could override this with a whole thing of adding a new path in your l4d2 files so the game treats your custom sound like it's a game file, but even then l4d2 cuts off all audio after like, 10-15 seconds? after you get in the saferoom, so i dunno if that would be worth it

One question: Do you know how to make music last a little longer? For example, the music I want to play lasts 37 seconds.

I think it could do with an increased volume. I can barely hear it.