The code added in http://hg.mozilla.org/mozilla-central/rev/4199176c083b means that a TrackUnionStream will read from input tracks samples that are before the start of the processing interval, and so may have been forgotten, zeroed. This could be corrected by shifting the position/timing of output samples instead of shifting the position of the read. However, the fix for bug 818822 will probably mean that all input samples will be aligned with graph processing intervals and so there is no longer a need to handle offset samples (from different rate streams).

If this wasn't fixed in bug 818822, then I expect it's fixed after changes in bug 1061046.