The only way I can think of is the DIY approach: find out their frequency (some frequency counters go down into the audio range), then use a 567 I.C. wired to that frequency. Long story short: when the 567 detects the right frequency, it can be used to mute the speaker. It's been around for several decades so it's a time-proven device, is multi-sourceable (so it's pretty much always available and costs only pennies). It's used often in DTMF tone detection/decoding.I listen on a BCD536HP, and the Metro Fire dispatch tones are piercing! Any way to quiet them?