You didn't mention your firmware version or build - "latest August 24" doesn't mean much since Roku rolls out new firmware versions over several months and there can be several builds @ once as well.
Make sure the following settings are configured for your 4640:
1) Enable passthrough: Settings/Audio/Digital output format = Passthrough
2) **Disable sound/volume modes: During playback (any app) press Options (*)/Sound Settings or Volume Modes/Volume Leveling & Dialog Enhancement = Off
** NOTE: Volume Modes or Sound Settings may not be supported on the 4640
Absolutely have seen this issue with volume leveling!
