Speak loud - what would be the opposite of that?
Hi everyone!
For you, what are the possibilites to say the opposite of "speak loud"? And what do these phrases MEAN for you?

1) Speak in a low voice.
-I read that "a low voice can be loud", so "speak in a low voice" wouldn't be the opposite of "speak loud". I'm afraid not everyone thinks the same, so that's why I'm asking this on italki :). Another English speaker said that "the most common sense would be for "low voice" to mean low levels of loudness". What do you think?

2) Speak quietly.

3) Speak low.
-Once, an American friend of mine corrected me when I said that (I wanted to say the opposite of "speak loud"). I wonder if it's only her way to see the thing or if other English speakers think the same way.

4) [here goes your way of saying it]

Thank you! :)
I would be glad if you correct my English mistakes ^^

Question

Speak loud - what would be the opposite of that?
Hi everyone!
For you, what are the possibilites to say the opposite of "speak loud"? And what do these phrases MEAN for you?

1) Speak in a low voice.
-I read that "a low voice can be loud", so "speak in a low voice" wouldn't be the opposite of "speak loud". I'm afraid not everyone thinks the same, so that's why I'm asking this on italki :). Another English speaker said that "the most common sense would be for "low voice" to mean low levels of loudness". What do you think?

2) Speak quietly.

3) Speak low.
-Once, an American friend of mine corrected me when I said that (I wanted to say the opposite of "speak loud"). I wonder if it's only her way to see the thing or if other English speakers think the same way.

4) [here goes your way of saying it]

Thank you! :)
I would be glad if you correct my English mistakes ^^

Answer

Speak in a low voice would be acceptable. "Speak softly" would also be another way of saying it.
If you're speaking to children you could say "use your inside/indoor voice". Don't say this to adults though since it would sound patronizing.
The simplest way to say it would just be "speak quietly" or "whisper".
I have never heard anyone ever say "speak low". You could definitely say "please lower your voice" though. That it a common one as well.
These are just ones I could think of off the top of my head.

Answer

1) and 2) can be used interchangeably in my opinion. I have never heard "speak low" used
4) when talking to a child, a common phrase is "use your "inside" voice" , meaning, when you are inside -- at home, at a store, etc), you need to lower your volume.