I read your manual and it looks like the volume of individual speakers can be adjusted in the 5.1 surround mode (L/R) but not in the B or C stereo only set up. The settings for the surround can be changed for BIG or SMALL speakers.
You could try various settings of BIG or SMALL to quiet the L/R when the outside speakers are on.
Also the impedance for the speakers are 4 ohms but up to 8 ohms is allowed. It is most likely your exterior speakers are 8 ohms so that to drive the outside loud enough to hear, you are over driving the interior speakers. The only way to fix it if that is indeed the case is to replace your outside speakers with 4 ohm ones so the apparent volume is the same. Remember we are talking about impedance not resistance (though both are measured in ohms) and adding a resistor will not work.
You could also place an additional 8 ohm speaker on each outside channel and hook them in parallel 8 over 8 to reduce the amp load to 4 ohms that way. See last line in link.
Impedance Matching of Audio Components
You could also install an impedance matching transformer in the 4 ohm outputs to match the effective impedance of 8 ohms at the speaker.
Impedance Matching Transformer 20 Ohms To 16 8 4 Ohms 10 W - Everything Else
If that won't work for you try installing a variable resistor from zero to say 8 ohms in parallel with the speaker to tune it to a ratio that works for you. Resistor 90W 0-8 OHM Var. Load | 28-1225 (281225) | Distributed By MCM
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