the decline of literature indicates the decline of a nation → φθίνει μὲν ἡ πόλις, φθίνει δ' ἡ μουσική | διαφθείρoνται οἱ λόγοι μόνον ὅσῳπερ διαφθείρεται ὁ δῆμος

[Jonathan]

I would suggest ἅμα τῷ τῆς πόλεως φθίνειν, φθίνει καὶ ἡ μουσική but I would welcome suggestions from others and yes, I have searched first! :)

[billberg23]

I like that you use ἡ πόλις for "nation," but don't think we can get away with τῷ τῆς πόλεως φθίνειν.  On the other hand, the perhaps more correct alternatives τῇ τῆς πόλεως φθίσει or τῷ τὴν πόλιν φθίνειν may seem awkward or grammatically misleading.  Non liquet, at least for that English translation of Goethe.
Goethe's original words were Die Literatur verdirbt sich nur in dem Maße, als die Menschen verdorbener werden — literally, "Literature is corrupted only to the extent that the people become more corrupt," which one is tempted to translate as διαφθείρεται ἡ μουσικὴ μόνον ὅσῳπερ διαφθείρεται ὁ δῆμος.  I use the verb διαφθείρομαι (“be ruined, corrupted”) for the condition that leads to the “decline” of literature.  I was inspired (as was Goethe, probably) by “Longinus” who, in De sublimitate 44, discusses a near identical situation — the decay of literary talent through indolence and apathy under Roman rule. The author reports a suggestion that it is democracy that produces true talent and the “sublime” style.  Without democracy (goes that argument), there is a λόγων ἀφορία (44.2 “dearth of literature”), for without democracy life is corrupt.  The word for that corruption in De sublimitate is διαφθορά, derived from the same verb I use:  … τελεσιουργεῖσθαι κατ' ὀλίγον τὴν τῶν βίων διαφθοράν, φθίνειν δὲ καὶ καταμαραίνεσθαι τὰ ψυχικὰ μεγέθη καὶ ἄζηλα γίνεσθαι (44.9) … (cf. also ἐν τῇ τοσαύτῃ λοιμικῇ τοῦ βίου διαφθορᾷ later in that paragraph). 

[Jonathan]

You are right, Billberg23. The subject of the infinitive must be accusative: so τῷ τὴν πόλιν φθίνειν. I prefer this to your alternative, since it keeps the verb and therefore gives you a better balance. If you wanted an even more sophisticated balance, you could have:  φθίνει μὲν ἡ πόλις, φθίνει δὲ ἡ μουσική, with an echo of Soph. OC 610:-φθίνει μὲν ἰσχὺς γῆς, φθίνει δὲ σώματος. Your alternative more discursive version, while accurate, is rather less elegant. As usual, I value your help.