I\'m writing an app where I record audio and upload the audio file over the web. In order to speed up the upload I want to start uploading before I\'ve finished recording.>
You will just have to use a container format that supports appending without editing the header.
I suggest Raw PCM samples. They can be converted to anything else at will.
Audacity will import raw bytes from the File -> Import -> Raw Data menu.
Another option might be a lossless audio codec such as FLAC inside of a streamable container format. (As done by a Czech web radio station)
I notice VLC can compress and stream FLAC-in-OGG from the soundcard. Should be a simple step from there to store or uncompress the stream on the server end.