A helicopter just flew low over my house .. we heard the transmission on my daughter's new am/fm alarm clock?
why is this? .. never happened before .. for the record it’s a hot wheels slam alarm ..
Thanks .. 
.. good answers .. first time it ever happened .. and they usually fly pretty low around here so it scared us all for a sec .. I was afraid they would have problems on their radio end too!
It’s not due to any quirks. With enough power, the radio transmits its signal on a certain frequency, Harmonics are also transmitted, which are at lower and higher frequencies. This particular phenomenon usually occurs with a different band radio. To give an example, If a person made a call on a 300 MHz radio, a VHF radio tuned to 150MHz will readily receive that lower first harmonic of the UHF radio. I used to hear the command post UHF radio on my FM radio in my office on a certain frequency, somewhere around 107 MHz, and the command post frequency was 321 MHz, the third harmonic.
This is the reason the old Emergency Beacons, and the ones the USAF still uses, radiate at 121.5 MHz and can be picked up on 243 MHz.
I think whatever helo was above your house was using a UHF frequency and she got a harmonic of it on her radio.
Not sure if you mean the geared transmission in the mechanicals of the helicopter or if you mean a radio transmission. Hearing a radio signal from that close encounter is a product of their strong broadcast signal reaching the radio and overwhelming the tuner with waves of energy. In some cases the wires going to the speakers act as antennas and the strong signal goes into the coil of the speaker cone causing movement we hear as sound.
Cheap radios are not known for tight frequency discrimination. Medevac and Police helicopters commonly use FM radio for dispatch and some of the frequencies are close to those used by radio stations. Being in close proximity to your home the signal from the helicopter was strong enough to bleed across channels. Its fairly common. Be happy you don’t have a "ham radio" operator living next door.
The VHF air band is between 108 – 136 MHz, which is right next to the FM broadcast band (88 – 108 MHz).
Close channels, and a very close transmitter (on the helicopter) make some interference possible in your situation. The people who design radios don’t usually plan for a transmitter just a few hundred feet away!
Typically Commercial Band FM Radios will pick up Aviation AM transmissions if they are quite close.
The frequency bands are quite far apart (88-108 MHz for FM radio, 118MHz to 137MHz for AM aviation radio) so it isn’t because they are right next to each other and ‘bleeding over’. 108MHz to 117.975MHz is used for aviation radio navigation aids, not (usually) voice transmission.
It is probably just a quirk in the circuitry when it is exposed to a powerful (relatively, due to the short distance) broadcast with a different modulation.
It happened because your house was under its flight path, on its way to its destination. So what?