That OpenMHA link is great. It's not specific to Teensy, but it is still good work.
If you want something that is more specific to Teensy, you should look at what the Tympan folks have done (I'm a founding member). Like OpenMHA, Tympan is...
Huge topic.
What kind of noise are you looking to reduce?
Noise Source: Real-world noise from your environment picked up by your mic, or electronic noise that's somehow electrically getting into your signal?
Frequency Characteristics...
If you were to use the OpenAudio library, you'd use this example...
https://github.com/chipaudette/OpenAudio_ArduinoLibrary/blob/master/examples/LowpassFilter_FD_OA/LowpassFilter_FD_OA.ino
It does a lowpass filter in the Frequency Domain. The...
As you're using a T4, you can use floating point operations. This means that you can use one of the floating point extensions of the Audio library, like the OpenAudio or Tympan libraries. These have options for steeper filter cutoffs, including...