During stage 3 of the Processing and Burning process, the burn button in iDVD may not spin during audio encoding. The thumbnail progress image may display a still image or a black rectangle (what's being displayed is either the last slide of the slideshow or the last frame of the movie whose audio iDVD is encoding at the time). The longer the movie or slideshow, the longer the pause will be while the audio is encoded.
Note: If you check Activity Monitor (/Applications/Utilities) and it indicates that some iDVD processes are not responding (such as encoding server), this does not mean that the DVD burning process has been interrupted. If you do check in Activity Monitor, also look at the "%CPU" column for iDVD. If the "%CPU" for an iDVD process is above 5, then iDVD is likely encoding.