What are the relative internal noise levels in various aircraft?
Let's start with the items for which hard data is easily available: 


This FAA chart gives relatively average noise levels for passenger jets, GA turbine aircraft, and helicopters. In addition, this chart should start to give an idea of how much noise is generated aerodynamically in aircraft: 


While this is older ground data and not cabin data, it still serves to show that the contribution of aerodynamic noise to total nose levels is not insignificant. 

Based on this, I would say that a likely ranking of these environments, from loudest to quietest, is:

1/2: Helicopters and uninsulated jets. In both cases, the close proximity to large, powerful engines leads to extremely high noise levels. 

3: General Aviation piston-engined planes. Again, these generally have little insulation,but smaller engines generating somewhat less sound. 

4: Passenger jet cabin. Protective insulation reduces sound, but actual noise levels vary based on seat position. 

5: Passenger jet cabin, gliding. As seen in the second chart, aerodynamic noise still contributes to a relatively loud environment.

6: Gliders. Slower speeds and cleaner aerodynamic profiles reduce aerodynamic noise, leading to a much quieter ride.