How can an animation be accelerated?

[6regor]

The game involves no movable bodies, only a background sprite sheet. The background is animated. The animation plays at a constant speed if no buttons are down. I would like to have the animation accelerate constantly with the press of one button and decelerate with the press of another button. Once a button is let go, I would like the animation to decelerate or accelerate accordingly back to the initial constant speed.

 

What is the best way to do this?

 

Thanks

 

P.S If this part is possible, I would like to then have the speed of the audio playing accordingly with the speed of the animation.