That not really because of kernel. From what I know of RTLinux it runs below everything else (or at higher priority if you will). Hence RTLinux will for example run it’s own stuff at higher priority then interrupt for networking or disk. Whatever the kernel, 32500 interrupts per second is high and is an extremly “expensive” way of generating sound.
If your really want to test the performance of the kernel set ticksize to 31us, run your sound program, then run another program that will act as benchmark. My guess is under QNX, that benchmark program will run faster then under Linux.
One of the way to reduce/eleminate the jitter would be to set the timer interrupt to be the highest but unfortunately that can’t be done unless you rebuild the startup code. (I’m guessing RTLinux does that by default)