From Gnash Project Wiki
The proprietary Nellymoser Asao codec was first introduced in Flash Player 6.
Flash Player allows users to encode audio using a microphone but the only supported format is Nellymoser Asao.
None? Flash 10 has Speex support, but it's unclear if users will be able to encode from their microphones into Speex format.
Technically it's junk, a sort of poor version of mp3 with less compression and worse audio quality. It's not patented; it is just secret.
Online Nellymoser-to-MP3 conversion sites such as flv2mp3.com work by running the audio into the FlashPlayer plugin and intercepting the audio data at the sound card. Archive.org has preserved a description and binary Debian sarge package of the flv2mp3 software but not the source code.
Nellymoser will sell you a decoder for $7500.
As of June 16, 2008, FFmpeg now includes an LGPL encoder/decoder. FFmpeg News This could be a starting point for Gnash.