Dual passive low-pass filter
Imagine the speaker coil had a centre tap. If you measured the voltage at the centre-tap relative to 0V it would be 0V with a balanced drive. Therefore you can split the problem in half and  imagine a grounded speaker with an impedance of 3.3 ohms and self inductance of 3.86mH.

Instead of C1 and C2 where they are, imagine just C1 connected across the half-speaker I have just described. Now proceed to calculate L1.

One word of warning - the 7.72mH self inductance of the speaker seems too high for my liking - I'm thinking that your RLC bridge has told you the effective parallel impedance (or maybe you meant 7.72uH)