Learn Hardcore Swahili: Mwishoni mwa sherehe, wimbo maalum uliimbwa na kwaya ya vijana. - At the end of the celebration, a special song was sung by the youth choir.

mwa sherehe is common and sounds quite natural/formal.
In many contexts, wa can also be used: mwishoni wa sherehe is understandable and some speakers do say it, though mwa sherehe is more idiomatic here.

In practice:

Learn mwishoni mwa … as a set pattern (e.g. mwishoni mwa wiki = at the weekend).
Don’t worry too much: natives will understand you with wa here, but mwa is the more standard collocation in this phrase.

How is the verb “uliimbwa” formed and what exactly does it mean?

uliimbwa means “was sung” and is the passive past form of -imba (to sing).

Breakdown:

u- = subject prefix for noun class 11 (agreeing with wimbo, “song”)
-li- = past tense marker
-imb- = verb root “sing”
-w- = passive suffix
-a = final vowel

So: u-li-imb-w-a = it (song) - past - sing - passive - FV → “it was sung.”

Why is “na kwaya ya vijana” translated as “by the youth choir” and not “with the youth choir”?

The word na can mean:

“and”
“with”
or “by” when used with a passive verb to introduce the agent.

In this sentence we have a passive verb (uliimbwa = was sung), so:

uliimbwa na kwaya ya vijana = was sung *by the youth choir*

If the verb were active (iliimba), then na kwaya ya vijana would more naturally be “with the choir” or simply omitted.

Why is “maalum” after “wimbo”, and does “maalum” change form for different noun classes?

In Swahili, adjectives normally come after the noun they describe:

wimbo maalum = a special song

About agreement:

Many common adjectives change form with noun class (e.g. mtu mzuri, watu wazuri).
maalum is a borrowed word (from Arabic) and is invariable: it usually stays maalum for all classes and numbers.

So both:

wimbo maalum (special song)
nyimbo maalum (special songs)

use the same adjective form maalum.

Why is it “kwaya ya vijana” and not “kwaya wa vijana”?

The connector -a (“of”) changes according to the noun class of the first noun (the possessed thing), not the owner:

kwaya is class 9.
The class‑9 form of -a is ya.

So:

kwaya ya vijana = choir of youths / youth choir
If the head noun were class 1, you’d see wa:
- mtoto wa vijana (child of the youths – odd meaning, but grammatically: class 1 → wa)

The key: “kwaya” (class 9) → ya, not wa.

Is “sherehe” singular or plural here, and does it have a different plural form?

sherehe is one of those nouns that look the same in singular and plural:

sherehe (sg.) = a celebration / a party / a ceremony
sherehe (pl.) = celebrations / parties / ceremonies

Context tells you whether it’s one event or several.
In mwishoni mwa sherehe, it’s naturally understood as one specific event (“the celebration” / “the ceremony”).

Could this sentence be written in the active voice, and would the meaning change?

Yes, an active version is:

Mwishoni mwa sherehe, kwaya ya vijana iliimba wimbo maalum.
= At the end of the celebration, the youth choir sang a special song.

Comparison:

Passive (original):
Mwishoni mwa sherehe, wimbo maalum uliimbwa na kwaya ya vijana.
Focuses on the song as the topic; the choir is background information.
Active:
Focuses on the choir as the doer.

The basic event is the same; the difference is emphasis and style.

Why does the sentence start with “Mwishoni mwa sherehe”? Could it go elsewhere?

Time/setting expressions are often placed at the beginning of the sentence in Swahili, just like in English:

Mwishoni mwa sherehe, wimbo maalum uliimbwa …
= At the end of the celebration, a special song was sung …

You could also say:

Wimbo maalum uliimbwa mwishoni mwa sherehe na kwaya ya vijana.

Both are grammatical.
Starting with mwishoni mwa sherehe simply foregrounds when the action happened.