Hmmm its still giving me some trouble. No audio yet. It seems like the code is fine but the program will not "pause" and play tmrpcm.play("birds.wav"); or any of the others because it only plays the first millisecond or so. Is this a valid hypothesis or am I crazy?
Maybe, but I could be crazy too, and you can't let crazy people decide whether you're crazy or not, can you?
The audio library is designed to run asynchronously, so the code will still execute while the music plays. It should keep playing however, unless you specifically stop the playback.
The IR library works fine with my Arduino Mega if the sounds are encoded at 8-11khz. Anything higher causes problems with the playback due to the excessive processing load. I don't have any other type of Arduino to test on at the moment but should work similarly on Uno, Nano, etc.
If the audio comes out like static, the most likely cause is some sort of file corruption on the SD card, or a problem with the file itself. If not, then make sure the wav file is encoded correctly (16khz, 8bit, mono). Also make sure you have the most recent version: https://github.com/TMRh20/TMRpcm/archive/master.zip
For the coding, take a look at MultiWii.com if you want an example of some solid open-source quadcopter coding for Arduino. Many quality commercial boards run this code, but it will work on a stock Arduino board with some sensor modules as well.
Not sure what you mean exactly by "internet/rc" control, but there are many options for wireless control, so it depends on what you plan to do with it. You can use a standard RC transmitter/receiver or even go totally custom and use a digital tranceiver, so you can communicate directly with other digital devices. (PC, Arduino, etc)
The Arduino will be able to catch very short triggers, and interrupts are about the most reliable way of catching short signals or pulses. Instead of having to check constantly if a pin is HIGH/LOW, the Arduino continually monitors it for you, and interrupts the main loop when the state of the pin changes. This leaves the CPU free to do other things in between signals.
Yeah, it can handle counting many thousands of times per second, so handling inputs from sensors on a bike should be no problem, especially if you use interrupts, so it only does work when there is a change of state.
Totally possible on one Arduino, and if speed is a concern, just make sure all of your modules support I2c or SPI, and it will handle it.
You can use interrupts for the hall-effect sensors, so the Arduino doesn't have to keep checking the state, and it will handle it easily enough. You woul have install jets on your bike before worry about overrunning its capabilities.
The amount of sensors is limited by what type of sensor etc, so is kind of impossible to give you a max number, but it can handle multiple I2C devices, so you could run the GPS, SD card and LCD all off I2C.
Basically, you usually want gyro, accellerometer, compass, and barometer sensors which you can get in a single module, then you just need something for wireless control, whether you buy a controller or build one. I used an old XBox controller.