Q. Is it possible to include OpenGL ES 2.0 support for older android devices?

No, I've been trying for many years to create the hardware accelerated SEGA Saturn emulation. My conclusion is that OpenGL ES 2.0 is not enough.

Q. Why can't this app play sound?

I know many glitches of sound. Currently(Dec. 2015), I'm focusing on stability and graphics.

Q. No background audio during gameplay.

It seems that you are using 'iso'. 'cue' is recommended for CDDA audio.

Q. Too slow to play. How can I do?