Many HD movie rentals on the iTunes store support true Surround Sound and contain a Dolby Digital sound track, but the iPod Hi-Fi does not support Dolby Digital sound. When Apple TV sends the digital file to the iPod Hi-Fi connected via an optical cable, the iPod Hi-Fi produces an audio rendition of the digital data. This creates the static audio with chirps and pops.
HD movie rentals on the iTunes store always have an AAC stereo audio track that is supported by an optically connected iPod Hi-Fi.
To get the AAC stereo audio track to play through your optically connected iPod Hi-Fi:
- On your Apple TV, go to Settings > Audio & Video.
- Select Dolby Digital Out and click Play to select Off.

This forces the Apple TV to output an available stereo audio track, rather than a Dolby Digital multi-channel audio track, over the HDMI and optical ports.
Note: Changing the Dolby Digital Out setting affects both the optical and HDMI audio ports.
If you want to output Dolby Digital on HDMI while the iPod Hi-Fi is connected, you must connect the iPod Hi-Fi via analog stereo cables instead of optical cables.