Methods, systems, and computer programs for monitoring quality of audio delivered over a communications channel are presented. One method includes an operation for defining timestamps. The timestamps are associated with a measure of time while delivering audio to a client computer, where each timestamp includes a plurality of timestamp bits. Further, the method includes an operation for modulating an audio signal with pseudo noise (PN) codes when a timestamp bit has a first logical value, and modulating the audio signal with a negative of the PN codes when the timestamp bit has a second logical value. After transmitting the modulated audio signal to the client computer, the timestamp bits are extracted from a received modulated audio signal to obtain received timestamps. The quality of the audio is assessed based on the received timestamps, and the quality of the audio is stored in computer memory.