You will drive yourself crazy trying to figure out "what the committee is listening for." There's simply no way to know for sure. On a positive note, the issues you bring up rarely come in to play until the finals. If you play with excellent pitch, rhythm, and have a solid, steady tempo that doesn't fluctuate all over the place, you are likely to advance. I haven't done a great deal of orchestral auditions; but, in the ones I have, it's painfully obvious in the early rounds who should be advanced. Usually, the issue is maintaining a steady tempo. Interpretation is more important in the later rounds.
Above all, play with confidence...let the committee know that you have studied the excerpts and this is how you interpret them. With almost no exceptions, play exactly what's on the page...you may often find that playing the tuba part removed from the orchestra is a different challenge than playing it with the orchestra. You will need to over-do dynamic changes a bit just to show you know they are there (and to demonstrate you can play with a good sound over a wide dynamic range).
As to tempo, I wouldn't try to simply reproduce what you have heard from previous recordings of a particular group. Rather, it would be a good idea to listen to a wide variety of interpretations from different orchestras and shoot for the middle. Really, the tempo won't be a huge deal unless you're way too slow or way too fast. Don't attempt to "dazzle" the committee with a very fast tempo on
Till Eulenspiegels lustige Streiche, for example...they won't care and brilliant fingers will rarely win you a tuba audition. Be precise instead, with a reasonable tempo. Again, keeping a steady tempo is much more important than the one you choose.
I hope that helps a bit...I hope some others with more audition experience than me will contribute. Maybe there is a secret out there that I don't know about...after all, I do sell auto parts for a living.
