I think audio files could have a lower size maintaining a good quality at lower bitrates (and sample rates but with lower changes) using OPUS format (with opus extension or ogg container) instead of AAC.
I made a test to support this using as basic information the constants here https://github.com/deltachat/deltachat-android/blob/master/src/org/thoughtcrime/securesms/audio/AudioCodec.java
I have used WAV and FLAC (without compression and lossless compression) as base files due a problem with audacity to export directly OPUS files.
Information about all audio files except base files, which only include the encoding pcm, are included in the name. I have made some others combinations specially since OPUS does not support 44100 hz of sample rate.
I have used 25kb as bitrate instead of 24 or 32 because I am still using the 0.304 version of Delta Chat Android whose audio files in my case are about 25 kb. 6kb is only to compare at lower bitrates and is officially supported by OPUS (and I would use it).