I wanted to be able to view a downloaded video in Flash video format (.flv) on my Windows Phone 7. This proved to be somewhat of a challenge. After installing the Windows 7 Codec Pack, I could open the file in Windows Live Movie Maker, but it froze when I tried to save it to Windows Media Video format (.wmv). So I searched around and found out that VLC Player could do the conversion.
My first attempt was to select Media->Convert/Save, select my input file, press the Convert/Save button, enter an output file with .mp4 extension and select profile Video – H.264 + AAC (MP4). The video was converted all right, but there was no sound. After running with messages turned on (Tools->Messages, Verbosity Level 2) I saw the following error messages: “main error: Failed to create audio filter” and “stream_out_transcode error: Failed to find conversion filter for resampling”. It turned out that the source sample rate was 48 kHz, but the target was 44.1 kHz, and VLC was unable to resample. (Tip: Use MediaInfo to see what your file really contains.)
So I had to change the profile to use 48 kHz sample rate, but in this case, since both the Flash video container and the MP4 container use the same audio format (AAC) I could just check Keep original audio track.
Save output to a Windows video library folder (e.g. “My Videos”) so that it shows up in the Zune videos collection. Now you should be able to sync it to the phone.