An update on this folks.
Thanks to the 150% increase in the JAR size limit, in the end, I was actually able to increase the quality of my sound files. Hurrah for Asha OS - it's ridiculous how this has managed to come to my rescue just in time
That didn't solve my issue with older Series 40 devices. However, thanks to some lateral thinking, I have gotten around this issue too. I had actually completely forgotten that J2ME supports the much more lightweight MIDI format. Since the original copies of my audio files were in MIDI format, I just used them.