Hi,
well, I use the core DNN captcha module, and there is no audio captcha with it as far as I know. I know that barreer free sites are more and more important, esp. in the European Union - but an audio captch would blow any security reason to use a captcha - it can be analyzed automatically, and therefore is not what is shoud be - a Completely Automated Turing test to tell Computers and Humans Apart.
Anyway: You may disable the captcha in the module settings if you want.
Best wishes
Michael