I've noticed an issues that a few others have confirmed. Sometimes, when decrypting engrams, picking up post master items, taking things from the vault..or even the odd gun shot sounds don't happen. I posted this out on a Destiny Community page on Facebook and a few other people have said they have found the same issue. Please note: it doesn't happen all the time, just every now and again. Sometimes it lasts a while and other times it goes away quite quickly.
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I've experienced the same thing except I've also lost voice audio as well. I was talking with Eris to which she instructed me to talk with someone else (doing the quest 2 times before, I know the 2nd character has talking), but when I talked with the 2nd person they had no voice audio at all.