This paper proposes a scheme for quantitative assessment of user-level (or perceptual) quality of service (QoS) for audio-video transmission by means of two psychometric methods: the method of paired comparisons and the law of comparative judgment. Moreover, we discuss QoS mapping from application-level QoS to user-level QoS by principal component analysis and multiple regression analysis. In the assessment, we simulate the transmission of an audio-video stream over a loaded network. In order to investigate the effect of the contents on QoS mapping, we treat two types of audio-video streams. By experiment, we demonstrate that our scheme can construct an interval scale as the user-level QoS parameter for each stream and represent it as a function of two application-level QoS parameters with high accuracy. We notice that the multiple regression line depends on the contents. We also propose the concept of control gain by media synchronization, which indicates how much media synchronization control subjectively lightens the average network load.