独り言 and 会話: an overhearer and a conversation partner
Compare muttering heard from the next room with conversations involving a friend or classroom practice. Hearing speech does not by itself make someone a participant.
Speech directed to oneself
独り言 (ひとりごと) means talking to oneself. 「考えを整理しながら小声で独り言を漏らした。」 describes quietly talking to oneself while organizing thoughts.
「隣の部屋から祖父の独り言が聞こえた。」 says the grandfather’s muttering was heard from the next room. A listener is present, but no responding conversation is stated.
Speech exchanged with a partner
会話 (かいわ) means conversation. 「友達との会話で時間を忘れました。」 says the speaker lost track of time while talking with a friend.
「授業で買い物の会話を練習した。」 describes practicing shopping conversations in class. Shopping is the practice topic; the sentence does not report an actual purchase.
Being within earshot and taking part
Self-directed speech can be heard by another person. Look for speech directed to a partner and exchanged responses rather than deciding from the reach of the sound.
The friend is a participant in the conversation example. In the next-room example, the grandfather is the speaker; the overhearer does not automatically become a conversation partner.
Does hearing the grandfather’s 独り言 from the next room establish participation in a conversation?
Show the answer and why
No. It establishes that the muttering was heard, but says nothing about the listener addressing the grandfather or replying.