Check your sound files. If you are using any custom sounds, maybe the ones for the yak are corrupted. Also, try upping a Yak offline and see if you hear your own engine sound. If it only happened this one time and not since, then probably a local and temporary resource issue on your end at the time. If you find you don't hear it when testing offline or if you play the sound files, then something is likely wrong with them (the sound files).
Just a suggestion.