What’s the phrase with the opposite meaning of “speak up, please”?

I would say "keep it down" is the opposite of "speak up". "Speak down" means to talk to someone as if they're your inferior, as in "Just because he's a waiter, doesn't mean you should speak down to him".

“lower your voice” or “quiet down”

"Please lower your voice" or "Quiet down" is what I would say. "Speak down" would not be used, and isn't a phrase where I'm from. "Talking down" or "Talk down to" is a phrase though, and it means to talk in a way that is condescending. Ex. "Don't talk down to me." to mean don't be condescending/rude when you speak to me.

"Let's use our *indoor* voices, children."

Pipe down

“Please lower your voice” would be a polite way to say this. Some commenters are suggesting “pipe down” but in many contexts that comes across as very rude (it’s somewhat similar to saying “shut up”).

To “speak down” to someone means to belittle them. For example, “I don’t like the way she speaks down to me; it makes me feel stupid.” So the opposite of “speak up” would be “quiet down” (be quieter).

I would say 'pipe down', but that's quite rude, like telling someone to shut up.

Pipe down. Also speaking down to someone means being condescending to them.

Pipe down.

"Shut up". Opposite in both meaning and tone.

“a little softer please?”. or “a little quieter?”

Keep your voice down

“Shut the fuck up”

"Can you please speak more quietly?" is what I'm used to lol

Never heard anyone say quiet down. Must be American.

“Ok… quieten down, please.”

"Speak up!" = Loudness. Volume. On your computer, you click the up-arrow to make things louder, and the down-arrow to make it quieter. On a TV remote control, "up" increases the volume, "down" reduces it. "Speak up" is like that. Please increase the volume; I can't hear you. --- There is another, unrelated thing; "talking down" at someone is being condescending. Saying things that make them look stupid.

Speak up = speak louder Pipe down/keep it down = speak less loud Speak down to = to condescend to Speak over = talk at the same time as someone or something else

Do you want the opposite or do you want to understand it, because the opposite will probably not help you understand. Quiet down please. Would be the inverse. Shut up would be the opposite. 

"Quiet down, please" is probably the most directly opposite phrase. As others said, to "speak down to" someone means to speak condescendingly to them. "Speak down" on its own doesn't really mean anything

Speak up, shut up.

To add a slang answer to go with the proper answers you've already received, someone might say "simmer down now." Normally, simmer refers to a kind of cooking with low heat. In this context, it can be used to mean "quiet down" or "calm down."

"Pipe down, you little yipster!"

Keep it down. Quiet down. Lower your voices. Ssshhhhhh. My mom used to say, “Pipe down,” but I feel that might be out of date.

kind option: keep it down.  mean option: shut up.  mean option extended: shut the fuck up no one wants to hear you talking. 

Pipe the FUCK DOWN.

"Keep it down" is an assertive way to say it and is usually used when you're very unhappy. It can come off as rude, especially if you sound like you're getting angry. "Please be quiet" and "Please keep it down" are both polite as long as you use a soft tone of voice.

Definitely pipe down.

'Quieter, please' often works without being rude 'Lower your voice please' is suitable For children 'use your indoor voice' is what in schools

“Why are you yelling?”

"Pipe down!"

“Lower your voice” works. But could be perceived as rude. If I was speaking with someone and wanted them to speak more quietly I would ask “could your lower your voice?” or “could you speak more quietly?” “Speak down” isn’t a phrase in English but “talk down to” means to belittle someone or speak in a condescending way.

"Speak up, please" and "speak down" are not opposites. "Speak up, please" is used to ask someone to speak louder. The opposite of this would be "lower your voice" which is asking someone to speak at a lower volume. "Speak down" or "Speak down to someone" means to talk to someone in a condescending way.

It means get a louder and pronounce words properly

Quiet, please

I think some of these replies are American and might seem a little rude to someone in the UK - "quiet down" for example, is something you might say to a classroom full of noisy kids. In the UK you'd just say "could you speak a little quieter please".

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Keep your voice(s) down- polite, something you would expect to hear in a library Lower your voice- I take that more to be a polite way of asking something to calm down when they are shouting because they are angry / annoyed / upset Say it don’t shout it- very informal, used when someone is talking unnecessarily loudly

“Quiet down, please.” There isn’t an opposite of “please” in this context.

“inside voice” is an option. Remind the person they are inside and don’t need to shout. This option can be a little rude if used with an adult you don’t know well, but it’s great with kids

“Please lower your voice” would be most polite. “Quiet down (please)” is less formal, and neutral politeness. “Be quiet!” and “Shut up!” would be pretty rude and have the connotation of “stop speaking at all”.

The opposite of "speak up, please" should be "Shut up, for f\*\*\* sake!" Just kidding.