Hi Martin Dreschau, and welcome to the Apple (user-to-user) Community!
Re: Can Voice Dictation be improved?
I don't know. There are some tips from Apple Support that may be helpful:
"On a Mac with Apple silicon, you can type text even while dictating; there’s no need to stop dictation. The microphone icon disappears while you type, and then reappears after you stop typing, so you can continue dictating"
"Ambiguous text is underlined in blue. For example, you may get the result “flour” when you intended the word “flower”. If this is the case, click the underlined word and select an alternative. You can also type
or dictate the correct text".
Above quotes come from: Dictate messages and documents on Mac - Apple Support
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Re: Can it learn from retyping misspelled words?
Seems Siri can learn from us, and improve things, yet I don't think Voice Dictation would be able to do that.
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Voice Control, if used, can give you more control, as you can move between different modes ... eg: command mode, spelling mode, and dictation mode:
How to: Use Voice Control on your Mac - Apple Support
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Re: voice dictation cannot understand my pronunciation correctly.
If you use Voice Control for dictation:
There's a (non-Apple) video online ... illustrating / explaining lots of helpful tips
eg: how to make corrections using your voice
towards the end of the video there are tips on how you can create your own vocabulary
https://www.youtube.com/watch?v=PWx64HznFiU
A quote from Use Voice Control on your Mac - Apple Support:
"Create your own dictation vocabulary
- "Open Voice Control settings, such as by saying “Open Voice Control settings” or “Open Voice Control preferences.”
- Click Vocabulary, or say “Click Vocabulary.”
- Click the add button (+) or say “Click add.”
- Type a new word or phrase as you want it to be entered when spoken.
Starting with the latest version of macOS Monterey, you can also import or export vocabulary."
All the best :-)