I've adjusted the playback rate for a video clip that includes audio(the little yellow diamond in the bottom left corner of the project window) to 0.83 and it previews fine. However, when I render the video, it seems to jump back to the default of 1.0. How can I get it to render at the 0.83 playback rate? Thanks.
The scrubber (that little yellow triangle) is only for setting speed of playback in the preview window. It has no effect on rendering. You could use a velocity envelope to adjust the speed of the video while rendering, but this doesn't lengthen the clip automatically nor does it adjust the speed of the audio.
A very simple way to achieve this is to caclulate the length your clip will be at this speed, then Ctrl-drag the end of the clip out to this length. This will slow down the audio and the video together. The audio will only be stretched in length though; the pitch will be preserved. You can right-mouse-button click on the audio part of the clip, choose Properties, and set the mode to "change length and pitch" if you want to have the pitch drop as well.