subvocal speech
Technical term in linguistics and cognitive science for internal speech without vocalization; often studied in relation to reading and thinking.
Subvocal speech is studied as internal speech during reading.
内言 is a broader term for inner speech, while 音声下言語 specifically refers to the subvocal, articulatory aspect.
Compound of 音声 (speech sound) + 下 (under) + 言語 (language), likely a calque from English 'subvocal speech'.