To furnish my thoughts, hacking on just the in-built media-scanner will not just suffice, as that requires rolling your own ROM! The reason being is that media-scanner is signed with a system certificate that is shared with the ROM in question.
And thus, that requires having the signed keys from the original ROM in order to replace the in-built media-scanner.
As to attempting to put MP3s in the
/system/media/audio folder - why? Are we talking about album mp3s which can creep up to between 5-8Mb in size? Or are we talking about Ogg Vorbis for ringtones/notifications?
/system partition is limited in size depending on the manufacturer's handset setup in terms of partition layouts.
If the audio media is stored on the SDCard under the respective directory
/sdcard/media/audio/notifications, along with a URI (Uniform Resource Identifier) pointing to the appropriate media declared as "external storage media" as in "MediaStore.Audio.Media.EXTERNAL_CONTENT_URI" (see the Developer Documentation on this), it will get picked up by Android itself and will show under the Settings > Sounds, as 'Phone ringtone' or in 'Notification ringtone'.
Depending on the ROM in itself, there may be a Development tools called 'Dev Tools', in there, there is an activity which can trigger the invocation of the in-build Media-Scanner, this is in stock/vanilla AOSP Android.