Is it possible to specify an equivalent RLC network for a loudspeaker?
Yes, but you need to determine what properties of the loudspeaker you want the RLC network to mimic (model).  You also need to specify the frequency range you are wanting the RLC network to be valid.

For example, if you want the network to model the electrical behavior of the speaker (e.g. have the RLC network have same voltage/current relationship over frequency) you would measure the speaker's impedance as a function of frequency and then design a circuit that has the same impedance vs. frequency.  For lower frequencies (i.e. below the resonant frequency), the speaker will look like a series resistance and inductance.  As the frequency increases, there will be resonances and the circuit becomes more complicated.  

Here is a picture from Wikipedia showing an equivalent circuit for a mechanical phonograph horn that illustrates what form an RLC network could take when resonance is taken into account.    

R, L and C are not frequency dependent; however, the impedance for L or C is strongly frequency dependent (\$Z_L=j2\pi f L\$ and \$Y_C=1/Z_C=j2\pi f C\$).