B wanted the audio from some f4v flash video files. Though I had flash video directshow codecs, the files were not opening in Virtualdub using AviSynth. Googled and found this page which guided me to the Adobe download site, where this explanation is given:
"Adobe Flash Media Server version 3.5 and later and Adobe Flash Media Live Encoder 3 can record content in MPEG-4 (F4V) format using an industry-standard recording technology known as "fragments" or "moof atoms." Some MPEG-4 compatible tools and players do not support moof atoms, and therefore cannot recognize files recorded by Flash Media Server. The F4V Post Processor tool aggregates the information from all the moof atoms into a single moov atom and outputs a new file. Use the F4V Post Processor tool to prepare F4V files for editing in Adobe Premiere® Pro, delivery over HTTP, or in video players that support H.264 and AAC formats, including Adobe Media Player."
So, I had to run the files through the f4vpp.exe and extract the audio from the post-processed files. The f4vpp process takes around 45 seconds for a 200 MB file and so on on my machine.