Před videohovorem zálohujte soubor a ztlumte mikrofon.

Because před normally requires the instrumental case here.

• videohovor = video call (dictionary form)
• před videohovorem = before the video call

So videohovorem is the instrumental singular of videohovor.

This is very common in Czech:

• před schůzkou = before the meeting
• před obědem = before lunch
• před videohovorem = before the video call

Here před means before in a time sense.

Czech před can refer to:

• time: before
• place: in front of

So:

• před videohovorem = before the video call
• před domem = in front of the house

The case after it is often the same, so context tells you whether it is about time or place.

Yes. Both are imperatives, so the sentence is giving instructions:

• zálohujte = back up
• ztlumte = mute / turn down

So the whole sentence is an instruction like:

• Back up the file and mute the microphone before the video call.

The ending -te marks the imperative for:

• you plural: when speaking to more than one person
• formal you singular: when speaking politely to one person

So zálohujte and ztlumte can mean:

• Back up / mute (speaking politely to one person)
• Back up / mute (speaking to several people)

If you were talking to one friend informally, you would usually say:

• zálohuj
• ztlum

So:

• Před videohovorem zálohuj soubor a ztlum mikrofon. = informal singular

The dictionary forms are:

• zálohovat = to back up
• ztlumit = to mute / dampen / turn down

So:

• zálohujte comes from zálohovat
• ztlumte comes from ztlumit

Because ztlumit and tlumit are related but not identical.

• tlumit is the basic verb: to dampen, reduce, muffle, turn down
• ztlumit is commonly used for a single completed action: to mute / turn down

In practical instructions, ztlumte mikrofon sounds natural for mute the microphone.

So the prefix z- helps make it sound like a completed action: do it now, once.

They are the direct objects of the verbs, so they are in the accusative case.

But both nouns are masculine inanimate, and in the singular, that accusative often looks exactly like the nominative.

So:

• soubor = nominative singular
• soubor = accusative singular too

and

• mikrofon = nominative singular
• mikrofon = accusative singular too

So the case is there grammatically, but the form happens to stay the same.

Because Czech commonly forms compound nouns as a single word.

So:

• videohovor = video call
• not video hovor

This is normal Czech spelling for many modern compounds.

Not completely. Czech word order is more flexible than English word order.

This sentence has a very natural instructional order:

• Před videohovorem = first, the time setting
• zálohujte soubor = first command
• a ztlumte mikrofon = second command

You could rearrange parts for emphasis, but this version sounds neutral and natural.

For example, Czech can move things around more easily than English, but not every version sounds equally normal in an instruction manual or interface.

Because Czech has no articles.

So Czech usually does not have separate words corresponding to:

• a
• an
• the

That means:

• soubor can mean a file or the file
• mikrofon can mean a microphone or the microphone

Context tells you which one is meant.

A rough guide is:

Před videohovorem zálohujte soubor a ztlumte mikrofon.

Approximate pronunciation:

• př is difficult for English speakers; it is a special Czech sound
• videohovorem has stress on the first syllable: VI-de-o-ho-vo-rem
• zálohujte has stress on ZÁ
• ztlumte starts with a consonant cluster ztl-, pronounced something like z-tlum-te
• Czech stress is usually on the first syllable of each word

A very rough English-style approximation would be:

PRZhed video-hovorem ZAA-loh-hooyteh SOW-bor a ZTLoom-teh MIK-rofon

But that is only approximate, especially for před.

You would usually switch the imperatives from -te to singular informal forms:

• Před videohovorem zálohuj soubor a ztlum mikrofon.

So the difference is mainly:

• zálohujte / ztlumte = formal singular or plural
• zálohuj / ztlum = informal singular

That is a very important distinction in Czech.