Ferrite Bead Current Rating
If the speaker is R ohms and is rated for P watts, then the maximum RMS current will be \$I_{RMS(max)}=\sqrt \frac{P}{R}\$

That will suffice for the current rating of the ferrite bead, but if you want to make sure it does not saturate you'll have to use a much higher number for the peak current.
PMPO is a ratio of 5~6 higher, so maybe 10:1. There's a bunch of guesswork here because the speaker rating probably takes that into account to some degree (I don't think a typical speaker can take continuous sine-wave inputs at the rated power for very long). 

If the amplifier is direct-coupled you can simply look at the power supply rails and divide that by R (and add a bit because the impedance is not constant).