May I? - Permission

The version in Answer 2 begins with a polite greeting, and then moves on, not only to the idea of helping (which, in the situation, should be obvious), but by asking HOW the assistant can help. This makes quicker progress than any of the other Answers.
Answer 1 is possible and quite widely-heard still, but it is short and rather 'off-handed'; it sounds almost automatic, and lacking in any 'human warmth'. Neither Answer 3 nor 4 is usual; these are the wrong auxiliary verbs, and too tentative.

Answer 1 is ridiculously florid; few, if any, serious English speakers would talk like that nowadays in such a situation.
Answer 2 is far from bad, but the version at No.3 puts the initiative more with the employee.

'Can' is a little stronger than the conditional 'could', and we did say 'please'!
('Could' offers the other people an escape, since it may not be possible for them to change the level of the sound; but in Answer 2 they either 'can or can't'.)

Either it is legal to do this, or it isn't; 'can' is the clearest way of establishing the fact of the matter. 'May' (Answer 2) suggests that you need permission from someone, rather than it being a clear matter of age under the law.

Answer 3 is the smoothest and least direct way of asking, but any of the others should get you an answer either way.

If it's an important question and you are a non-native speaker (and perhaps somewhat nervous about all this), this Answer should still 'work for you'. But in a sensitive situation, any of the others would offer a gentler way into the topic.

This one is the best, because (although it is leading towards a question) it is, itself, in the form of a statement ~ which it might be slightly harder for the other person to resist.
Answers 1 & 4 are each better than No.2, which is really quite blunt and abrupt. Answer 1 puts a personal and emotional aspect onto the question (the other person may refuse, on the grounds that they are too busy or 'not in the right mood'). With Answer 4 there is also the open chance that the other person will not have enough time, or pretend that they haven't.

If you ask 'Why not ... ?' (Answer 1), the person may think of some awkward, contrary reason not to follow your suggestion ('It's too expensive / the remote control is broken / I can't be bothered', etc.); 'How about ... ?' is clear, yet indirect and almost impersonal.
Indeed, rather than just using the standard language-learner's all-purpose phrase ('I'd like ... a chocolate ice-cream / to play tennis tomorrow / a puppy for Christmas', etc.), how about using 'How about ... ?' when you want to make a friendly suggestion? 'How about a game of tennis?' 'How about a trip to town this evening?'

A fairly dramatic 'How COULD you?' is the classic line (or, about someone else: 'How could she?' etc., with the rest of the sentence left unspoken, since, presumably, it is all too obvious what the person has done to cause offence).
'How DARE you?' (Answer 2) is at least as strong, but there is a difference, because this one suggests disobedience.
In a classic novel (or period drama on film or television), a major female character 'of a certain age' ~ a duchess, let's say ~ might ask a younger male relative 'How dare you?' when he has become romantically involved with someone that she has specifically forbidden him to see (in which case, how did he have the courage to go deliberately against her wishes?). She might ask him 'How could you?' if he has done something cruel or distasteful, like stealing a pig from the neighbouring estate and coming home in filthy clothing with it in the small hours of the morning.

This is perhaps just a little 'clever' (Answer 2 would also be fine), but your English friends will probably congratulate you on your good manners if you use this one!