Having a more fine-grained control over the speed of the audio playback would surely more user-friendly.
YouTube does a great job here. They provide buttons and a slider:
One thing I think would be worth having is the ability to increase and decrease the speed by clicking buttons, along with the option to configure the size of those increments. For example, YouTube uses a default step of 0.05 and doesn't let the user change it.
I use a Chrome Extension called "YouTube Playback Speed Control" which lets me change the size of the increments. It provides me with buttons on the screen to increase and decrease the playback with minimal clicks.