This has happened to me as well. I was next to my friend and we were watching another player shoot his mosin from 200yds away and we could not hear his shots. Then I shot my longhorn and I could hear my own shot but he was unable to hear the shot. This problem persisted thought the match until we re logged.

Closing as duplicate of #0011461: No gunshot sounds on hardcore/first person

Please vote up the original issue to increase visibility, add any extra information in the notes section, and monitor it for further developments.