Frankly the question as asked cannot be done as changing the bit rate must always require re-encoding and reducing the bit rate will almost always reduce the quality of the track - the exceptions will be when the track had already been processed to increase the bit rate above that which was originally recorded but not always in this case or for simple waveforms that fit the new bit rate as well as they do the old.
Think of it as changing the size of the pages in a book, unless you reflow or scale the text and images you cannot do it and retain all of the original information unless you are lucky enough that all of the page contents fit within the new page dimensions without change because there happened to be large borders.
There are a number of GUI based Audio Editors that can change the bit rate on save or export and support .M4A format, necessarily re-encoding the format but not many that I know of that support batch processing.
- Audacity allows you to create a 'Chain' of operations which can include exporting with given settings and apply them to the opened file but opens a window for each file so still requires a fair amount of manual intervention.
- VLC Media Player may well be able to do this but it certainly isn't clear how.
This brings us back to a not GUI option FFMPEG which can do this with ease and with minimal loss of quality. This can easily be input into a batch file containing something like:
REM UNTESTED!
for fn in $* (
ffmpeg -i %fn% -c:a aac -b:a 160k fixedrate/%fn%
)
That you coud then drop your files onto.
There are a number of FFMPEG GUIs such as AVAINTI & FfmpegYAG that are worth checking out.
