All my SI (cakewalk) (bass, piano, etc.) soft synths have no sound when I assign them to a MIDI channel. Dimension is the same way.  No problem with 3rd party (Toontrack, etc.) though.
 
I know this is vauge, ask any questions you may have and I can fill in the answers.
 
Thank you and God bless,
Craig

What channel?
What patches on the soft synhts. Channel 10 is for percusion/drums

This can happen to Dimension Pro if you didn't register it. After the registration period expires (30 days?), it stops making sound until you enter the registration code.

Dimennsion Pro has been registered with Cakewalk.

Soundcard? ASIO or WDM mode? 

You have to select a sound from the list. The default is no sound. 

More info on how your operating Dim pro and your settings for the midi and audio track