What is the opposite of “Could you talk a little louder”?

In a conversation, when I don't hear someone, I usually say:

Could you talk a little louder please?

However, what should I say if:

• Someone is being very loud in the other room when talking on the phone.
• Someone is making too much noise.

I am looking for a polite expression. Any suggestions?

No matter how you say it it may come across as bossy, since the other person (in the situations you mention) isn't talking to you. I'd probably use:

"Would you tone it down a bit, please."

An alternative that is a little more formal, "Could you speak a little softer?" You could also follow it up with a reason or a thank you which would soften the tone. For example, if it's a conversation you're not involved in, "I'm having trouble hearing [someone]." If it's a conversation that you're involved in, you can also try speaking more softly yourself. A more conspiratorial statement would be, "Hey, let's not tell everyone our business," followed by you dropping your voice.

Could you lower your voice?

Please lower your voice.

Please talk quietly.

The exact opposite of louder is softer. Simply say: "It's too loud, could you speak a bit softer?"