Started my prototype of these notifiers and tied it to OpenHAB:
1) Dog Bark Sensor
Using a $3 sound sensor similar to this one, you can sense loud sharp noises. The ones with the larger diameter diaphragm might work better for sensitive sound detection than the smaller ones. The one I have is a digital output device, and you use the pot to change the trigger level.
2) Gas / Smoke Sensor
You can get a MQ2 sensor for about $7. It's sensitive to several different types of combustible gas, including LP (propane cylinders) and methane gas from household gas stoves. The properties of the gas dictate the best location for the sensor. Sensing smoke is best done from above. Methane also floats, so mounting this sensor high up would work well. If you're trying to sense propane, mount the sensor down low since propane is heavier than air. They have both analog and digital output for the amount of explosive gas.
3) Flame sensor
Flame sensors are a few dollars each. They have both analog and digital out.
Putting it all together on a prototype board, this what I have. This is going to be a wall-powered device, since the MQ-2 gas sensor uses quite a bit of energy to keep its internal heating element hot. The wireless transceiver is the the same RFM69HW that I've been using for all my other wireless Arduino sensor projects.
The OpenHAB screen has alarm indicators and alarm enable switches. You have to enable the bark alarm before loud noises on the sensor will set off notification emails. Same with fire and smoke/gas. The "Auto" function will enable and disable the notifications based on work schedule, without you having to mess with it.
I need to polish up the Arduino sketches and do more testing. Also need to determine the minimum flame size needed to set off the flame sensor. Initial testing shows that a fairly large fire is needed to set off the flame sensor. I still have to write the OpenHAB logic to set the auto enable functionality. I also need to fine-tune the bark sensor sensitivity and placement. It seems pretty sensitive to orientation.