Seems this effect doesn't work. Or if it does, the hailing of our custom NPC maxes your skill anyways making it useless.
I tested it by doing this:
Bard 1.5 epic (+22 modifier, no AA singing masteries yet), using Bellow of Chaos song on Vlan the dummy:
281 damage each time I test consistantly.
Cast renonance on myself via clickie, cast Bellow again:
281 damage each time consistantly.
click off resonance, cast Amplification (a singing mod boost song):
334 damage each time consistantly.
put on resonance + cast amplification:
334 damage each time consistantly.
Conclusion: Resonance clickie doesn't work.
Relevance/Importance? Not a ton, just noting.
Suggested Change? For now it still is a good stat item. If you intended this to have a nice clickie, I guess we can figure something out.
Another finding: Talking to Tremmo and saying "bard" after being a lvl 70 maxxed out bard lowers your instrument skills down to 200, the cap is actually 235 for bards @ lvl 70. (Well maybe more than 235, will tell you when I remaster my skills haha)