Isn't there a simple adverb for the opposite of 'loudly'? [closed]

I am wondering, isn't there any brief and common adverb (adjective plus -ly) in English that would enable me saying, e.g.

The transcript is imperfect because over lengthy periods people were speaking very xxxxx-ly

meaning: they did not speak loud enough to be intelligible on the record. They spoke with too low voices. Of course, I could say "... are speaking with too low voices", but I find this inconvenient: why use three words when one could do?

In German, I would simply say "... weil sie zu leise sprechen". I am a bit puzzled that in English I need to use complicated expressions for such a simple thing as the opposite of "speaking loudly".

I suppose, saying "they are speaking lowly" would be misleading. Right?

Note: there are related questions in English.Stackexchange, but none of them focussing my exact problem, as far as I see.

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

• Does "speak in a low voice" refer to volume/loudness or to pitch?

• Right word to represent "speaking with low audible voice"?

Later addition:

I am adding information here because the question got closed with a notification on alleged lack of preliminary research. Well, so, here are my preliminary attempts to come up with a solution based on a) my active dictionary of English b) various other dictionaries.

• quietly - Problem: my understanding is, that "quietly" carries a positive connotation: It would be an indicator of being a person of good upbringing to speak quietly (instead of loudly); in my context of audio records of research interviews it is however a negative thing to speak too quietly. Please correct me if I am wrong.

• muttering/mumbling: these describe certain ways of speaking, usually of course in a low voice, but also with e.g. a lack of physical movement of the speach organs. In my context "muttering" or "mumbling" would be over-specific. Moreover, I fear that using such words I would offend my client (to whom I want to communicate the reason for the imperfect transcript). I do not want to tell him "You were mumbling" because a) he was not actually mumbling, he simply spoke with very low voice and b) even if he did it would be too confrontational to use this very word, I believe.

Solution 1:

The German phrase "weil sie zu leise sprechen" can be directly translated as "because they were speaking too softly" or "too quietly".

The opposite of loud is quiet and quietly is a perfectly valid adverb.

Solution 2:

You could say inaudibly

not audible; incapable of being heard.

other synonyms might be softly or quietly

or indistinctly

Not clear or sharply defined

Solution 3:

Faintly could work.

In a faint or almost imperceptible degree; very slightly; in faint tones; without vividness or distinctness.

"They were speaking faintly," or "they were speaking in faint tones" would both portray your meaning of the opposite of "loudly." Or you could even use "faintly" to describe how you're hearing them, as in "speaking so you could faintly hear."