System Information
Win7/64, GTX 750 Ti

Blender Version
2.74 official,
2.75-42314b3-win64

Short description of error
If a speed control effect strip (addendum: or other effect strips that have at least one input strip) is muted, it is still considered and rendered as black or transparent.

Exact steps for others to reproduce the error

1. Open
   VSE Muted Speedcontrol.blend61 KBDownload
2. As you can see in the top-right preview, the output is black even though the speed control strip is muted.
3. If you render (F12), the output is all transparent

Instead, I expect a muted strip to be ignored completely, in this case resulting in rendering only channel 1, which is the scene strip.