ipcrm. If that happens, have a look in your
~ cat /proc/sysvipc/sem key semid perms nsems uid gid cuid cgid otime ctime 5678293 163840 660 1 1000 29 1000 1000 1201199302 1201198890
The only semaphore listed here is the one we're interested about. Kill it with the following command-line (probably only works as root):
~ ipcrm -S 5678293
And then, magic ! Sound works again !