Hello!
Im back with some news on this topic.
I am still using Cubase Elements 7 (7.0.80), and I still have troubles changing the the tempo of some recorded media (WAV, 44KHz, 16 Bit, recorded within Cubase).
This is what I experienced:
If the Tempo Track contains a change in signature, e.g. changing from 6/8 to 4/4 and then back to 6/8 (like in ‘Where The Streets Have No Name’ from U2), the tempo of the media - in my case - could not be changed after it had been recorded. 
Solution
Since I will continue using the Tempo Track in this project, it was necessary to do the following:
1.) removing the media completely from my project (no re-building of the WAV with Audacity this time)
2.) saving, closing and re-opening the project
3.) re-importing the media into the project
Kind regards