Methods and apparatus to monitor mobile devices are disclosed. A disclosed example method involves operating first and second wireless telephone device terminals. In the example method, the first wireless telephone device terminal includes a first media decoder configured to decode a first media encoding format and the second wireless telephone device terminal includes a second media decoder configured to decode a second media encoding format. The example method also involves receiving a first media presentation via the first wireless telephone device terminal and a second media presentation via the second wireless telephone device terminal. The first media presentation is encoded using the first media encoding format and the second media presentation is encoded using the second media encoding format. The first and second media presentations include substantially similar media content. The example method also involves generating a first reference signature of the first media presentation and a second reference signature of the second media presentation. The first reference signature is stored in association with first device identifier information indicative of the first wireless telephone device terminal. The second reference signature is stored in association with second device identifier information indicative of the second wireless telephone device terminal.