Getting the track mapped to follow the varying tempo is a rather easy thing. After the mapping, the tempo grid positions are all at the correct position. Now, the 2nd step is where stall. After all it is quite simple, just want to slow down the whole project. On the software side all it'd had to do is multiply the tempo map with a number. (smaller than 1 to slow it down).
There is the Edit->Tempo Scaling menu and inserted a number, pressed ok, but the audio track tempo did not change, although the retain musical position is checked for "audio object positions". Is that a bug or intended behavior?