Poor management on your part imo.
Working in a team isn't just about the obvious work side of things, a cohesive team doesn't have niggles like this occurring and building into worse resentments because good office managers nip them in the bud.
You can't make the 2 slackers make teas, but you can pull them up for their general lack of team spirit and point out to them that such behaviour affects the overall atmosphere for everyone. Also that you don't take a favour if you have no intention of returning it! That should be basic good manners their parents taught them, given they clearly weren't it now falls to you.
Imo it's a valuable part of their training. I've worked in many offices, and these "little" niggles can and do affect morale and working relationships negatively.
I'm not a big cuppa drinker myself but I would never accept cuppas and not EVER make them myself when it operates as "rounds" as it clearly does In your office (not every office does but many do).
If everyone hums a little, that's fine but if someone "ups the ante" by singing and that's annoying/distracting then that too needs nipped in the bud. Plus imagine the cacophony if everyone started singing!
Your job is to deal with things like this BEFORE it gets to resentment/complaint levels tbh I think you also need to consider and observe if there's other issues/difficulties occurring between the complainant and the main one being complained about - are they slacking in other ways? Are they not respecting the more experienced staff? The complainant may be reluctant to "drop them in it" over something important but the resentment over other concerns is being voiced instead on these issues? In other words is it REALLY about the tea?
The directors need to make their own damn cuppas though! Those attitudes should have gone out with the ark! Bet they never make for anyone!
"it's just the way it's always been" is the WORST reason ever for doing something at work!
"if you partake you make." Yep!