Tempo Change screwing up audio clips!

OK - changing my clips all to "absolute" time base solved the current issue. How do I set SONAR to use that as the default for all audio clips?

The M:B:T ruler is the fixed visual time reference in SONAR. It doesn't change when you change tempo; the Now time just speeds up or slows down accordingly. This means audio clips will appear to change length when you change tempo, but their absolute playback tempo and duration won't change.
 
What does change with tempo is the absolute start time of both MIDI and Audio. By default, the timebase for both audio and MIDI is Musical time; it affects only the start time not the absolute length/duration of the clip. This means the start time of a clip will follow the tempo change. An audio clip that starts at 9:01:000 will continue to start at that measure. This makes sense in most instances, and is what you would want if you were enabling autostretch on audio clips so they can actually follow the M:B:T tempo change.

I'm pretty sure you cannot change the default timebase. But depending on what you're wanting to accomplish with changing the tempo, you might want to try using Set Measure/Beat At Now instead of manually entering tempo changes. SM/BAN automatically maintains absolute timing of both MIDI and Audio as it inserts tempos to fit the timeline to the performance.