After spending some time solving these issues, here's a short guide to possible solutions, as worked for me:

Only MIDI sound comes out, while guitar/mic/wave sounds do not:

• Go to all of your MIDI plugins, click Route and select: Mix
  Now the MIDI output will be mixed with all other outputs. Otherwise, it will just take over the channel.
  

(Thank you Stefanha for this tip!)

Only channels 1-2 on your audio interface are audible, while the others are not sent to the session:
This is a, somewhat cumbersome but working, workaround that I found and that works perfectly well for me.

• Remove all of your MIDI plugins from Jammr.
  
• Run standalone version of your MIDI plugins, outside of Jammr. For example: your Piano module, as a standalone application.
  Make sure the output is set to go through the same audio interface you plan to use for all the other instruments, not any other. Let's call this one: “the Input Audio Interface” for now on.
  
• Go to your audio interface settings (on the Input Audio Interface) and check the Loopback option. I use UR-44 (by Steinberg), which has that option in the dspMixFx control application (see attached image), but this is a standard feature that should exist for your audio interface as well. Use the documentation to find it.
  
• Go to Jammr, File/Settings/Audio and set the Input Device to your Input Audio Interface, the same one you use for all of your instruments- digital or analog.
  
• In the same page, set the Output Device to a different device(!!), for example- the built-in audio interface in your computer, or a separate USB headset. Let's call this one: “the Output Audio Interface” for now on.
  This is to prevent the other players' music to be looped back to them in delay, by your looping back audio interface… If you don't do that, they'll definitely complain
  
• In the same page above, you can check the Play back my audio, then turn down the output volume on the Input Audio Interface (with all of your instruments connected) and turn up the volume on the secondary, the Output Audio Interface (the one you have your headphones connected to).
  
  The result would be that all sound produced via the Input Audio Interface would be mixed down into channels 1 and 2, respectively, which Jammr can detect and send to the other jammers, while the sound coming in and your own sound being routed to the Output Audio Interface, mixing them down together.
  
• Reconnect to the channel. Now everything should be audible and you can play and be heard.
  
  Note that the next segment would make sense only if you did not go through this one.
  

You're not doing the above steps, and the sound comes out “crackly”, as if it's going through a blender…

• It's all about loopback… You hear the sound once when it's being produced, and other times when it is echoed by Jammr.
  
• Go to the File/Settings/Audio in Jammr and uncheck the Play back my audio
  

Hope the above helped you jam!

Eric

Thanks for posting!