What is causing this Op-Amp Distortion
Since you're new to electronics I simplify here a little bit:

OpAmps don't have the current drive capability to drive speakers directly. Speakers usually have a impedance in the low ohms. 8 Ohm is typical. 

A ordinary OpAmp won't drive much more than 50mA to 100mA. At 10V Signal that is roughly equal to an impedance of 1000 to 2000 Ohm. Much higher than your speakers.

If you connect speakers to your OpAmp output, the OpAmp will likely try to drive the speakers nonetheless, but after reaching the current limit it won't drive it any further. That will clip the waveform, and this is what you hear as distortion.

You need a power amplifier stage to drive your speakers. These can be built using an OpAmp and some extra transistors to handle the higher load. 

On the other hand it's more economic if you just use an audio amplifier chip that handles the 15 watt directly. The TDA2030 chip for example will happily drive 10 to 14 watt and is really cheap.

In short: Think of OpAmps as pre-amplifiers, to drive anything bigger you need a power amplifier.