Playing Macintosh System Sounds

1
Dear fellow PTB users,

To play a beep, I use the following command:

SND('Play',sin(0:0.3:4000).*0.75);

and then I can vary loudness, frequency, etc. by altering the values.
To play a system sound, I do the following:

SND('Play','Wild Eep');

or

SND('Play',Droplet');

My problem here is that I cannot vary the loudness of these system
sounds. Does anyone know if it possible to vary these sounds?

Thanks,
Larry James