Larmet är igång.

Because Swedish usually puts the definite article on the end of the noun.

• ett larm = an alarm
• larmet = the alarm

So larmet is the singular definite form. Since larm is an ett-word (a neuter noun), the definite ending is -et.

Är is the present tense of vara, which means to be.

So:

• jag är = I am
• det är = it is
• larmet är = the alarm is

In this sentence, är links larmet to its state: igång.

Igång describes the state of the alarm. After är, it means something like on, active, running, or in operation, depending on context.

A useful way to learn it is as a chunk:

• vara igång = to be on / to be running / to be active

So Larmet är igång means the alarm is in an active state.

For a learner, the safest approach is to learn igång as part of the common expression vara igång.

You will also see it in other very common combinations:

• komma igång = get started
• sätta igång = start, set in motion
• vara igång = be running / be on

So even if you think of its historical parts, in modern learning it is usually best treated as a fixed expression.

This is the normal Swedish main-clause order:

subject + verb + complement

• Larmet = subject
• är = verb
• igång = complement

Swedish also follows the verb-second rule in main clauses. That means if you move something else to the front, the verb still stays in second position:

• Nu är larmet igång. = Now the alarm is on.

Notice that är still comes second.

Put inte after the verb:

• Larmet är inte igång. = The alarm is not on / active.

That placement is very typical in simple Swedish main clauses: subject + verb + inte + rest

Swap the verb and subject:

• Är larmet igång? = Is the alarm on?

This is the standard yes/no question pattern in Swedish: verb + subject + rest

Not always. Larm can refer to an alarm in several related senses, depending on context:

• an alarm system
• an alarm signal
• an alert
• a warning/alarm being raised

So Larmet är igång could mean the alarm system is activated, or that the alarm is currently active, depending on the situation.

Not exactly.

• Larmet är igång usually focuses on the alarm being activated/on/in operation
• Larmet går usually means the alarm is going off / sounding

So if you mean the siren is actually making noise, Larmet går is often more specific.
If you mean the system is switched on or active, Larmet är igång fits better.

These are the most useful forms:

• ett larm = an alarm
• larmet = the alarm
• larm = alarms
• larmen = the alarms

So larm has the same form in singular indefinite and plural indefinite, which can feel unusual to English speakers.

A rough guide is:

• larmet ≈ LAHR-met
• är ≈ a short vowel sound that does not match English perfectly
• igång ≈ ee-GONG, with the stress on the second part

So the whole sentence is roughly:

LAHR-met ehr ee-GONG

That said, Swedish vowel sounds are important, so it is best to listen to native audio if you can.