One thing I've done with sound has been using Audacity to convert to mono, increase the volume, and apply some dynamic compression before importing into Decker. It helps a bit since the 8-bit sound of Decker has a much higher noise floor than normal 16-bit audio.
I haven't tried it but since the Decker audio is 8khz sample rate, it may also help using Audacity to downsample to this rate before importing as maybe it does a better job than Decker's built-in functions.
Inherently it's going to sound kinda fuzzy/degraded though since that's the nature of 8khz 8-bit audio as compared to like 44.1khz 16-bit audio.