I have never seen such a motto in historical instruments so I say nothing about that (and I second Andrew’s perplexities about Italian harpsichords mottoes, but I’ll leave this to more expert people than me).
However, I’d be surprised if the motto was to be found in any historical instruments, though, as it’s wrong, language-wise. Andrew is right.
The Svetonius original is indeed: “[…] prodire in scaenam concupivit, subinde inter familiares Graecum proverbium iactans occultae musicae nullum esse respectum”, which is to be translated literally: “[…] he wished (ora craved) to get on stage, often reciting to his family a Greek proverb which says that no care is given to hidden music”.
The part of the sentence, “occultae musicae nullum esse respectum” is called an “infinitive”, and indeed “esse” is an infinite verb, “to be”. It’s used with “verba dicendi”, such as “to say, to declaim, to tell”, etc. In Italian there still is an infinitive sentence, albeit very formal, but I don’t think there is one in English, so it’s difficult to translate literally. However, the sentence “occultae musicae nullum esse respectum” would be translated as “to be no care for hidden music”, which is plain wrong. In Latin, a standalone sentence should be plainly “Occultae musicae nullum est respectus”.
Yes, “respectus” and not “respectum” because “respectum” is right in the infinitive sentence (which has the verb in the infinite form and the subject in accusative), but in a plain sentence the subject goes in nominative and the verb in indicative. (sorry I use the Italian/Latin words for the verbs and substantives forms because I don’t know exactly how they are in English).
As for “respectum”, it could mean “respect”, but it’ first meaning is “care”, and moreover the substantive “respectus” derives from the verb “respicio”, “to look after, to care for”.
Sorry for the long message, probably useless for the Latin-savvy reader.