Unfortunately, Titanfall is still in development, so you are likely to encounter issues like that.
From what I've found out, sound bugs occur in Titanfall and plague other players too, so there is probably nothing wrong at your end. Some guy recommends
disabling sound enhancements.
If that doesn't help, you might try to lower the sound acceleration.
Other than that I'd recommend to just wait it out until it gets fixed by the devs. But considering it's Titanfall, that's probably the hardest.