changing project tempo stretches audio clip?

Audio doesnt follow the project tempo unless you enabled it as a groove clip. So just disable groove cliping and it wont change when you change the tempo.

The reason audio appears to stretch/compress when you change tempo is that the M:B:T timeline is the fixed visual reference in SONAR.

If absolute time were the fixed reference, then the Now time cursor would travel at a constant rate, and you would see measures and beats expand and contract when changing tempo, while audio remained fixed along with the timeline. But since the M:B:T ruler is fixed (at any given zoom level) and SONAR represents changing tempo by changing the speed of the Now time cursor, the audio has to be re-drawn to reflect the changed relationship. But as Beagle pointed out, it's only visual, the underlying audio file isn't affected.