Cell app to measure watts when driving my speakers

My amplifier is rated at 44 watts per channel, with the first 6 watts in class A. I would like to know if there is an App application to use, like a decibel meter app, to check how many watts is my amplifier putting using with different volume levels.

I tried to be as clear as possible.

Regards,
 
You can't measure the electrical power with a phone, just the acoustical sound level.

You could roughly estimate the power with a scope connected across the speaker.
With a sine wave, six watts into 8 ohms is about 7 Volts RMS, or about 10V peak.
 
You could use a 400Hz sine generator, and measure the AC Vrms across the speaker.
Don't drive the speaker too hard. Then at a given volume level, roughly the power = V x V / 8 ohms.

If your meter were a true RMS type, you could use music instead of a sine wave, and have a better measurement.