When you open Sound in Control Panel and click the Recording Tab, what is displayed? Should look something like this.
Also look at page 93 of your manual. Settings under Crystal Voice.
Noise Reduction and Acoustic Echo Cancellation. Try enable/disabling these settings and retest each change.
Pay particular attention to the settings for Mic and Line in on Page 95-98
Open the Sound Blaster Recon3Di control panel and
go to the Mixer page. In the REC section, click the Input
Device icon and select Rear Line In. Later you can use
the Line-In slider to set the volume.
This should have a direct impact (raise or lower) gain for Mic/Line input.