Yeah, it was available for 5.9. I just tried to install it using an app I wrote some time ago to migrate packages, and it installs 1.4.13 along with jack and another lib it needs.
There are newer versions, and the 5.9 slackbuild does not write a correctly named tlz, but just in case you want to check configure options, or, you want the .desktop icon, here's the url where you can find previous package source: http://vectorlinux.osuosl.org/veclinux-5.9/source/extra/x-apps/mhwaveedit/
It'd be good to migrate the slackbuild to our 'standard' format, but it's your decision
Btw, thank you for your effords packaging apps for your new distro. It's much appreciated.