Others may offer helpful and more specific critique, but here are my thoughts on attenuators in general.

The "T" configuration is not necessary, since you want a very low output impedance of the attenuator (10 ohms). I would use a simple voltage divider. The series resistance (called R1) goes from the output of the sound card to the output of the attenuator. The shunt resistance (called R2) goes from the output of the attenuator to ground. The output voltage of the attenuator is across R2. For an approximate gain of -60dB (which is 0.001 in V/V) you would make R1 equal to 10K ohms and R2 equal to 10 ohms. In general, the loss of an attenuator is not simply the ratio of R2/R1, but is R2/(R1+R2). In this case, since R1>>R2, we have R1+R2 approximately equal to R1, hence, the gain is approximately R2/R1. The impedance looking back into the attenuator output is given by R2//(R1+RS) where RS is the impedance driving the attenuator input and the symbol // means "in parallel with"). Because the output resistance of your soundcard is 100 ohms, the attenuator would have an output resistance of 10 // (10K + 100) or about 10 ohms.

By the way, R1 // R2 is equal to 1/((1/R1)+(1/R2)).

Tom