I am actually producing some screen captures with sound (topic ASP.NET 2.0) for microsoft germany. I wasted a lot of time to get the "best" results. So here is my experience.
1) use a good micro or headset. Dont save money.
2.) use Windows Media Encoder to record your blogcast. Its free!
http://www.microsoft.com/windows/windowsmedia/download/
3) create a file with eg name "bgdefault.wme" and the following content
<?xml version="1.0"?>
<WMEncoder major_version="9"
minor_version="0"
Name="WMEncoder785"
SynchroniesOperation="0" >
<Description Author="Hannes Preishuber (hannesp@ppedv.de)"
Title="Blogcast Settings File 1024x768 16bit Colour Depth"
Copyright="ppedv AG"
Description="your blogcast description" />
<SourceGroups >
<SourceGroup Name="Navigation" >
<Source Type="WMENC_VIDEO"
Scheme="screencap"
PixelAspectRatioX="1"
PixelAspectRatioY="1" >
<UserData >
<WMENC_STRING Name="WindowTitle" />
<WMENC_LONG Name="CaptureWindow" Value="0" />
<WMENC_LONG Name="Left" Value="0" />
<WMENC_LONG Name="Top" Value="0" />
<WMENC_LONG Name="Right" Value="1024" />
<WMENC_LONG Name="Bottom" Value="768" />
<WMENC_DOUBLE Name="FrameInterval" Value="0,1000" />
<WMENC_BOOL Name="FlashRect" Value="no" />
<WMENC_BOOL Name="Screen" Value="yes" />
</UserData>
</Source>
<Source Type="WMENC_AUDIO"
Scheme="device"
InputSource="Default_Audio_Device"
ContentMode="1" >
<UserData >
<WMENC_BOOL Name="UseRecordQueue" Value="no" />
<WMENC_STRING Name="RecordTempPath" />
<WMENC_STRING Name="MinimumDiskSpace" Value="20971520" />
<WMENC_BOOL Name="SaveToDiskOnlyDuringCapture" Value="no" />
</UserData>
</Source>
<EncoderProfile id="Voice quality audio (CBR) / Screen capture (CBR)" />
<UserData >
<WMENC_LONG Name="Video0\CropMethod" Value="0" />
<WMENC_LONG Name="Video0\CropCustomT" Value="0" />
<WMENC_LONG Name="Video0\CropCustomL" Value="0" />
<WMENC_LONG Name="Video0\CropCustomR" Value="0" />
<WMENC_LONG Name="Video0\CropCustomB" Value="0" />
</UserData>
</SourceGroup>
</SourceGroups>
<File LocalFileName="c:\BlogCast.wmv" />
<WMEncoder_Profile id="Voice quality audio (CBR) / Screen capture (CBR)" >
<![CDATA[ <profile version="589824"
storageformat="1"
name="Voice quality audio (CBR) / Screen capture (CBR)"
description="">
<streamconfig majortype="{73647561-0000-0010-8000-00AA00389B71}"
streamnumber="1"
streamname="Audio Stream"
inputname="Audio809"
bitrate="4000"
bufferwindow="8000"
reliabletransport="0"
decodercomplexity=""
rfc1766langid="en-gb"
>
<wmmediatype subtype="{0000000A-0000-0010-8000-00AA00389B71}"
bfixedsizesamples="1"
btemporalcompression="0"
lsamplesize="150">
<waveformatex wFormatTag="10"
nChannels="1"
nSamplesPerSec="8000"
nAvgBytesPerSec="500"
nBlockAlign="150"
wBitsPerSample="16"
codecdata="100004000000000000000000000000000000A7801A0000125A92A4EB800000000000000000000000000000000000"/>
</wmmediatype>
</streamconfig>
<streamconfig majortype="{73646976-0000-0010-8000-00AA00389B71}"
streamnumber="2"
streamname="Video Stream"
inputname="Video809"
bitrate="86000"
bufferwindow="8000"
reliabletransport="0"
decodercomplexity="AU"
rfc1766langid="en-gb"
>
<videomediaprops maxkeyframespacing="80000000"
quality="100"/>
<wmmediatype subtype="{3253534D-0000-0010-8000-00AA00389B71}"
bfixedsizesamples="0"
btemporalcompression="1"
lsamplesize="0">
<videoinfoheader dwbitrate="86000"
dwbiterrorrate="0"
avgtimeperframe="1000000">
<rcsource left="0"
top="0"
right="0"
bottom="0"/>
<rctarget left="0"
top="0"
right="0"
bottom="0"/>
<bitmapinfoheader biwidth="0"
biheight="0"
biplanes="1"
bibitcount="24"
bicompression="MSS2"
bisizeimage="0"
bixpelspermeter="0"
biypelspermeter="0"
biclrused="0"
biclrimportant="0"/>
</videoinfoheader>
</wmmediatype>
</streamconfig>
</profile>
]]>
</WMEncoder_Profile>
<UserData >
<WMENC_LONG Name="VideoSize\ResizeMethod" Value="3" />
<WMENC_LONG Name="Encoding\Bitrate0\Video0\CustomW" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate0\Video0\CustomH" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate1\Video0\CustomW" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate1\Video0\CustomH" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate2\Video0\CustomW" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate2\Video0\CustomH" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate3\Video0\CustomW" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate3\Video0\CustomH" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate4\Video0\CustomW" Value="0" />
<WMENC_LONG Name="Encoding\Bitrate4\Video0\CustomH" Value="0" />
<WMENC_LONG Name="Encoding\Dest" Value="2" />
<WMENC_LONG Name="Encoding\Type" Value="1" />
<WMENC_STRING Name="Encoding\Audio0" />
<WMENC_STRING Name="Encoding\Video0" />
<WMENC_STRING Name="Encoding\Script0" />
</UserData>
</WMEncoder>
Use it as template for the settings (simply double click)
4) if you make errors ( not the smal ones) stop the recording and start a new recording with a new file. Start at a defined screen position where everything was fine and you made a speech break.
5) Use the Windows Media File Editor to cut of the "wrong part " of every video.You must save it with a new name. Be carefull with timing and listen several time on the sound track.
6) download Windows Media Format 9.5 SDK
http://msdn.microsoft.com/windowsmedia/downloads/default.aspx
7) extract MWVAppend.exe and use this commandline tool to append 2 videos in a new named one. Repeat this step for every video you want to append. Take care that you can only append if it is the same format.
The result is that my video use ~3 MB for 18 minutes