A method of determining the number of recipients for an audio content item includes receiving a packet associated with the audio content item at a network monitoring device. The monitoring device includes a packet inspection module that analyzes a data payload of the packet to identify the audio content item. In an embodiment, the packet inspection module identifies the audio content item by determining an audio signature based on the payload, and comparing the signature to a set of known signatures. Once the audio content item is identified, an indicator is recorded to indicate that the content item was communicated. By aggregating the recorded indicators, the number of recipients for the audio content item is determined.