ankooru ni ohako wo utai, tomodati to hamotta.

Yes—normally 十八番 would be read じゅうはちばん and literally mean number 18.

But in this expression, 十八番 is a special word read おはこ, meaning:

• one’s specialty
• one’s signature piece
• the thing one is especially good at

In a singing or karaoke context, 十八番を歌う means to sing your go-to song or best song.

So this is an important vocabulary item to memorize as a set:

• 十八番（おはこ）

It is not something you could easily guess just from the kanji.

Yes, basically.

The particle を marks the direct object of 歌う. Even though 十八番 literally means one’s specialty / signature piece, in this context it refers to the song or performance being sung.

So 十八番を歌う means:

• to sing one’s signature song
• to sing one’s specialty number

Japanese often lets a word like 十八番 stand on its own without adding 曲 or 歌.

Here, アンコールに means something like:

• for the encore
• during the encore
• at the encore

The particle に marks the occasion or point at which something happens.

So the idea is that the singing happened as the encore performance.

A learner might expect で, and sometimes で can sound natural in similar situations, but に here gives the sense of:

• doing something on the occasion of the encore
• choosing something for the encore slot

So アンコールに十八番を歌い is like saying:

• I sang my signature song for the encore.

歌い is the continuative/stem form of 歌う, and here it is being used to connect to the next action:

• 歌い、友達とハモった。

This is a written-style or narrative way to link actions, similar to:

• 歌って、友達とハモった。

So the meaning is basically:

• sang ..., and then harmonized with a friend

Compared with 歌って, 歌い can sound:

• a little more written
• a little more concise
• slightly more literary or narrative

It is not the polite ます-stem here in the sense of politeness; it is the verb’s linking form.

ハモった is the past casual form of ハモる, which means:

• to sing in harmony
• to harmonize

It comes from harmony and is a very common casual word in music and karaoke contexts.

So 友達とハモった means:

• harmonized with a friend
• sang in harmony with a friend

It sounds natural and conversational.

The particle と here means with in the sense of doing something together with someone.

So:

• 友達とハモった = harmonized with a friend

This is the normal particle when two people do an action together.

Compare:

• 友達と歌った = sang with a friend
• 友達に歌った would sound very different, more like singing to a friend

So と is marking a companion/partner in the action.

Yes. Japanese often omits the subject when it is clear from context.

So this sentence does not explicitly say:

• I
• we
• he/she

The most natural assumption in many contexts would be I, especially if someone is talking about their own performance.

But depending on the larger context, it could also refer to someone else.

This is very normal in Japanese.

It is casual/plain style.

You can tell because the final verb is:

• ハモった rather than ハモりました

Also, the sentence has a conversational feel because of words like:

• アンコール
• ハモる
• 十八番

A more polite version might look like:

• アンコールに十八番を歌って、友達とハモりました。

The comma just helps readability and shows a pause between the two linked actions:

• sang my signature song,
• and harmonized with a friend

It is not doing anything very special grammatically. The connection is mainly created by 歌い itself.

So the sentence would still make sense without the comma, but the comma makes it easier to read.

Often, yes.

In everyday conversation, many speakers would more naturally say:

• アンコールに十八番を歌って、友達とハモった。

Using 歌い is not wrong, but it can sound a bit more like:

• written narration
• stylized storytelling
• compact descriptive prose

So if you are speaking casually, 歌って may feel more natural. If you see 歌い in writing, though, it is completely normal.