The importance of considering the time-variant factors in the evaluation of multimedia telecommunication quality is discussed from the viewpoints of future multimedia networds and services. The quality degradation caused by bit erorrs in LD-CELP and ADPCM codecs was measured under conditions of variable speech-sample duration, a varying number of errored bits, and variable error-burst duation. The degradation in the mean-opinion score (i.e., the DMOS) for communication with variable speech-sample duration was quantified for both the number of errored bits and the error-burst duration. The results due to the number of errored bits were significant at a significance level of 1% under all conditions, as well as due to the interaction between the number of errored bits and the error-burst duration under some conditions. The effects of the quality factors were different among codecs. For example, the longer the error-burst duration, the better the quality in LD-CELP, while in ADPCM the duration did not have any effect. These experiments identified several items that should be considered in developing a quality-evaluation method, including determining such testing parameters as speech-sample duration, a measure for expressing degradation, and a procedure for subjective evaluation.