独り言を

talking to oneself

→ 独り言
phrase Uncommon #17,916
talking to oneself Usually followed by 言う, meaning “to say” or “to speak,” as in 独り言を言う (to talk to oneself).
JA: 彼女は独り言を言いながら歩いていた。
EN: She was walking while talking to herself.

トムは独り言を言い始めた。

Tom began talking to himself.

独り言をいいます。

I'm talking to myself.

メアリーはどうしようかと独り言をいった。

Mary wondered aloud what she should do.

トムは独り言を言っている。

Tom is talking to himself.

独り言を・・・放校はひどいけど自分のせいよ

Just talking to myself... Being expelled is harsh, but it's my own fault.

— サスペリア (1977)

私は彼がぶつぶつ独り言をいうのを聞いた。

I heard him muttering to himself.