No, nothing was connected to the device's USB port when I recorded the audio with noise/glitch. The device was inserted into the sleep cover during this recording though.
You're right, it seems like the audio files are not encoded with MP3, but MP4 (despite the files being named with a .mp3 extension). If you're interested in the details of the file format/encoder, K-lite's MediaInfo reported this:
Code:
General
Complete name : D:\DesktopSaulo\RecordedWithExternalMic-PerfectAudio(2021_11_22_11_34_36).mp3
Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp42 (isom/mp42)
File size : 38.0 KiB
Duration : 19 s 460 ms
Overall bit rate mode : Constant
Overall bit rate : 16.0 kb/s
Encoded date : UTC 2021-11-22 03:34:56
Tagged date : UTC 2021-11-22 03:34:56
com.android.version : 11
FileExtension_Invalid : mov mp4 m4v m4a m4b m4p 3ga 3gpa 3gpp 3gp 3gpp2 3g2 k3g jpm jpx mqv ismv isma ismt f4a f4b f4v
Audio
ID : 1
Format : AMR
Format/Info : Adaptive Multi-Rate
Format profile : Narrow band
Codec ID : samr
Duration : 19 s 460 ms
Bit rate mode : Constant
Bit rate : 12.8 kb/s
Channel(s) : 1 channel
Sampling rate : 8 000 Hz
Bit depth : 13 bits
Stream size : 30.4 KiB (80%)
Title : SoundHandle
Writing library :
Language : English
Encoded date : UTC 2021-11-22 03:34:56
Tagged date : UTC 2021-11-22 03:34:56