For the purpose of streaming, you must convert audio files to the Windows Media streaming format (WMA). The file size of the file depends on the type of profile you select when converting to Windows Media. Profiles with lower bit rates will produce smaller files.
Click on Tools] Settings ] General Settings. Select "WMV - For Windows Media Server (Streaming)" from the "Convert to Format" dropdown. Then select the "Properties" tab. From "Audio Profiles", select a profile with the least bit-rate. Since sermons are usually voice content, you can select "Voice quality audio (CBR)". For audio files, Digital Media Converter will ignore the Video Profiles selection.
You can read the following article that explains streaming media format in details:
http://www.deskshare.com/Resources/articles/vc_StreamingMediaFormats.aspx
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