Comment 7 for bug 206315

I have found the cause and offer an interim solution:

The cause seems to be in the default volume levels assigned by 'alsamixer'. The master volume is set quite high (80%), causing the very loud system sounds. The 'PCM' and 'Front' levels were also set to 80% (it seems that this is 80% of the 80% master volume), making any PCM sounds relatively quiet. This affects anything played with 'aplay' (such as the GDM sounds), but not those played by the nautilus previewer or totem, for example.

The solution is to increase the 'PCM' and 'Front' volume levels in 'alsamixer' to 100%, and reduce the master volume level to a lower value (I suggest 50%, but whichever you are happy with yourself). These can be done from the command line thus:

$ amixer set PCM 100%
$ amixer set Front 100%
$ amixer set Master 50%

Of course you can also do it manually using 'alsamixer' or 'gnome-volume-control'.

P.S. In response to Daniel's question above, I have now tried it in 8.10 and it is reproducible there too.