|Most likely the decompiler is unable to extract the audio in its original state and simply converts it to a mp3 with as the high quality as possible (swf supports more audio formats than mp3).|
I tried myself with Sothink SWF Decompiler 7.1 and it could extract it to a 22.0 kHz mono wav at 9.57 MiB. If you zip or gzip the wav (using Deflate at Normal) you get a good idea how much space the file would take inside a compressed swf container, then the wav became 9.3 MiB in size for me. It means the extracted wav (or the mp3 that you got) could not have been how the audio was stored in the swf, because swf doesn't have better compression than Deflate @ Normal.
Well, there is a new compression added to a later version of swf but swfchan doesn't support it right now and since the flash you linked to is on swfchan the old compression is used in the swf. swfchan vill get support for the "newer" compression in time, luckily almost nobody uses it so few swfs are affected.
Just for fun I tried compressing the wav audio file in a couple more formats (at their highest settings):
.rar (classic) = 8.10 MiB
.rar (newer rar5) = 6.75 MiB
.7z (LZMA) = 3.5 MiB
Would have been great if swf used LZMA compression instead of simple Deflate right from the beginning, the algorithm wasn't really finished then however and perhaps added too much decompression overhead for computers at the time.
In fact that "new compression" later added to swf (introduced in 2012 with Flash Professional CS6) is LZMA. It's however not at all at the same level as 7-Zip's compression using LZMA:
I tried sticking the wav into a swf and set the audio export settings to Raw 22kHz mono (raw audio in flash is PCM, which is the same format used by the wav container) and with the old Deflate compression the swf became 9.29 MiB (notice it's pretty much the same as the 9.3 MiB zip) while with the new LZMA compression the swf became 8.38 MiB. With only 0.91 MiB gained in this case it's no wonder LZMA swfs never really caught on since you don't gain that much but will break compatibility with ~10 years of tools and upload sites developed for flash.
Anyway, you can't really accomplish the level of compression that you seek with flash. Unless of course you write some really nice ActionScript 3 decompressor that basically does what 7-Zip does and you store the externally-created LZMA bytes inside the code (the flash you linked to just uses the plain audio stuff that flash provides by default).