I wouldn't say QoS manipulates data. At most, there might be inspection, and even that is unlikely to be done by a switch. For audio applications, QoS can be useful if the network is congested. In such a case, tagging frames carrying audio data with a high priority ensures that switches will only drop them as a last resort, and they might also spend less time in various queues. None of this matters unless the network is operating near maximum capacity. In a home environment, this is extremely unlikely to be the case.