to generate multiple specific tone frequency, i'll really prefer to use the 16 bit internal timer.

Since you know your frequency range... it's a bit easy to calculate the according period you'll need. Use this period to get the according timer interrupt and VOILA.

OR have a variable external Oscilator and vary the frequency using a digital potentiometer like those microchip MCP42010 or else...

I figure SOUND statement is mostely use to drive piezo transducer.