If you can't find a Mac app with a good "voice" then recording the audio from the Fire with your Mac. You'll need a range of cables/adapters to do it, but it can work.
With most modern Macs you need:
- a stereo patch cord, 3.5mm male-to-male, that to connect to your fire
- a stereo-to mono adapter (female stereo to male mono)
- audio recording software (e.g. Garage Band, Audacity, ...)
The only big issues will then be getting the audio out to be low enough volume so that it doesn't overload the sensitive mic input cause distorted audio. Start with the Fire's volume at zero and raise it slowly while monitoring the recording from the Mac.
If your Mac has an earphone only jack instead of the earphone/mic combo jack or the old pair of discrete jacks then you would need a USB audio device and appropriate cords. Some of these devices are simple stereo headphone out and mono mic in and cost less than $10USD. Others are more substantial and offer higher quality audio in and out and input is stereo. These will be more in the $30-60 range, and up, and are often mixers in addition to being audio interfaces (DACs).