I thought I was the last person in the world still using 3.1.3 before I updated a few months ago.
We did make some changes in the last release that require iOS 4.0 including touch-tone sounds on the Keypad and inline voicemail playing. I can submit a feature request to consider adding legacy support for people still using v3.x, but the intention was to drop support for anything older than 4.x altogether. Even if the audio was changed to support 3.x, there will definitely come a time when it just isn't feasible to regression test that far back and account for all the subtle differences.
We'd like to hear from the community if there's a lot of desire to support iOS 3.x, and this would be the perfect thread for that. Anyone want to +1 that request? Also, are there particular reasons for sticking with 3.x or is it just a matter of "haven't gotten around to upgrading" at this point?