Unfortunately, the whole point of GUI sound is that it is played along with the GUI layer.
Using
PlaySoundAtEntity is your best bet.
You can
modify the volume in your .snt file
or perhaps even
edit the source .ogg file with some software like
Audacity.
You can also play the sound at "Player" instead of an entity to make it as loud as GUI sound. (if the .snt file doesn't cap it)
Now, Mudbill's solution
DOES work. But I would argue that it's not the cleanest way. Consider using PlaySoundAtEntity with your sound tweaked a little in a single line instead of timers.