WMV File Extension

Windows Media Video: Windows Media based file which contains both video and audio 
File extension: .wmv
MIME type: video/x-ms-wmv
Uniform Type Identifier: com.microsoft.windows-​media-wmv
Developed by: Microsoft
Type of format: video file format

MP3 File Extension

MPEG-1 Audio Layer 3
File extension: .mp3
MIME type: audio/mpeg
Type of format: Audio

Extract audio from WMV to MP3 with

Mini-stream Ripper or ASX to MP3 Converter