The MultiSound Classic has internal audio connectors that you might have 
overlooked.  

These are 4-pin "Berg" connectors that are seated in between the In, Aux, 
and Out external connectors.  

The upper connector (between the External In port and the external Aux port) 
acts the same as the In port.  

The lower connector is the same as the external Aux port.  

Orient the board facing you with the slot to the right.
The wiring is as follows: 

__________________ 
External IN Port  |
__________________|

 = = = =   Left|Signal Ground | Right | Chassis Ground (Internal IN) 
__________________
External Aux Port |
__________________|

 = = = =   Left | Signal Ground |Right | Chassis Ground (Internal AUX)
__________________
External Out Port |
__________________|

(See page 56 of the manual for wiring information.)