Increase/decrease playback speed in rhythmbox

Solution 1:

RBPitch

What you describe fits nicely into the capabilities of an external plugin called RBPitch.

From the launchpad description:

rbpitch is a plugin for Rhythmbox that lets you change the pitch, tempo, and speed in real-time while playing a song. The main features of rbpitch include:

*Change the pitch and tempo of your music independently of each other.

*Change the "speed", which is pitch+tempo scaled together without aliasing.

to install - for RB2.96 & RB 2.97

sudo apt-add-repository ppa:smcnam/rbpitch-release
sudo apt-get update
sudo apt-get install rbpitch

In Rhythmbox, go to the Plugins manager via the Edit -> Plugins menu. Make sure the checkbox next to "Pitch and Tempo Shifting" is checked.

RB2.98 and beyond

There hasn't been any code changes since the RB2.97 release - thus this plugin is only applicable for Ubuntu 12.04 & 12.10.

Considerable changes to the Rhythmbox menu structure etc has been made since RB 2.99 - so whilst it may be possible to switch your sources back to 12.10 to get it to run on RB2.98/RB2.99 and RB3.0, I'm doubtful. Its probably better to throw an issue onto the OP's issue tracker to see future plans.