Why does my AI voiceover sound robotic?

Long clauses delivered flat sound robotic for reasons unrelated to the synthesiser. If a sentence could survive being read aloud by a person without sounding written, the problem is the writing. Split it.

Numbers are the other common source. Raw digits get rendered unpredictably. Write them the way they should be said, or use markup that tells the synthesiser to treat them as a number, date or currency.

Proper nouns are the third. A synthesiser that has never heard a name will pronounce it consistently wrong, and no amount of post-processing fixes a wrong pronunciation.

Real speech has pauses at commas and longer ones at sentence boundaries, and a synthesiser with no pauses produces the flat cadence people describe as robotic. If your provider supports pause markup, use it. If not, paragraph breaks produce some of the same effect.

Rhythm is also a matter of sentence length. A series of similarly sized sentences reads as a list, whatever the voice quality. Varying the length reads as speech.

An und ducked music bed masks the frequencies the voice needs, and the result reads as cheap regardless of how good the voice is. Sidechain the music down several dB under speech and back up between lines.

Loudness matters here too. A master well below the platform target gets turned up at playback, which raises the noise floor and makes everything sound worse at exactly the moment it is played.

Genuinely, most "bad AI voice" complaints resolve in the mix rather than in the voice. It is worth checking which you have before changing providers.

Modern neural TTS is genuinely good and also genuinely detectable by a determined listener on careful comparison. Nobody is at the level where a listener cannot tell, and claims to that effect are usually marketing.

The practical consequence is that if authenticity is the requirement, a synthetic voice may not be the right choice. For short-form where the content carries the value and the delivery is functional, it very often is.