Same problem just started 5-13-2021.
I have 6 Roku Express + devices.
All are Model # 3910RW
One of them has a serial number of YG001Y087366
Software version 10.0.0 build 4195-51
Device ID C53951087366
Roku Media Player Version 5.5 build 9
Some of my video files play correctly.
The video files that have either no sound or that irritating crackle sound are mainly Matroska but I do have some MP4 video files that have the same problem.
All of the files are stored on a Seagate 8TB desktop hard drive connected to a Netgear WNDR4300 DLNA certified router.
ALL of these files worked without a problem prior to this EXCEPT when Roku Media Player had that last major change. That time it was almost completely broken.
I would really appreciate a way to go back to the old build while the problem is being fixed.