Version: 20.6.0 (revision 9418) (64-bit) on Windows 10 Pro x64
SMplayer is set as Windows default for videos so whenever I open any video via explorer or any other file browsers, SMplayer is launched and starts playing video - but there's no audio.
A quick DOUBLE toggle of muting (hitting the M key TWICE) starts audio output, so it's not that SMplayer is actually invoked muted.
Playing a number of videos that way (i.e. invoking SMplayer several times for several videos in a row) has exatly the same effect.
Actually invoking a 2nd video while playing the 1st one (with sound re-enabled) changes the video shown in SMplayer but again the new video starts with no sound.
I recently migrated from a Windows 7 x64 Laptop to a Windows 10 x64 Desktop and I had no such problem with the same version of SMplayer on the laptop.
Given, it's not a major issue but an inconvenience but still I would guess that's not as it should be.
Apart from the change of operating system there are of course hardware differences between both platforms, one being that the current desktop provides 3 different sound outputs:
1) the most often used Realtek chipset on the main board
2) the alternate Logitech Headset that comes with its own USB sound card
3) the nearly never used nVidia High Definition Audio supplied by the GPU/ Monitor combo
The laptop had only 2 different devices available, the on board Dolby system and the external Logitech USB sound card and actually used the latter nearly 100% of the time.
I just mention them as obvious differences, though the fact that a simple muting and un-muting getting it going doesn't seem to indicate that SMplayer is having a problem to decide for 1 of the 3 output devices.