mosfet n type as switch

[quaserdog]

HI there, i have project that i am working on using a n type mosfet connected to speaker output from a weather station i have removed the speaker and connected to the mosfet to switch a relay, my problem is that even though i have measured the voltage when speaker activated at 3.1volts but both negative and positve wave form (this on a scope) this doesn't switch the mosfet is there some clever electronics person out there that could prehaps describe (a) if this will ever work with some modification or (b) some other way to get the relay to operate when speaker is active ie in alarm condtion on weather station thanks in advance have sorted that problem using a diode and electrolic cap which charges up enough to switch the mosfet thanks
regards kevin