(In reply to lethalwp from comment #27)
> After more tests, i have a small race condition on my systemctl setup which
> causes X to be unable to load the "amdgpu" driver and fallbacks on
> "modesetting" instead.
> 
> In this case, the videocard can't use vdpau anymore, so it's fully software
> decoding. Then the sound is playing perfectly.
> 
> When X is loading the amdgpu driver, the sound problems are still present.
> The video must eat the sound bandwidth or something (?)

Software decode/display may load CPUs enough to hide the alsa bug I mentioned
earlier.

Try testing with an audio only file (be sure the player for this test is using
alsa not pulse).