The author uses the base form of the verb "be" because it is a subjunctive construction.
The subjunctive in English is used to form sentences that do not describe known objective facts. These include statements about one's state of mind, such as opinion, belief, purpose, intention, or desire. It contrasts with the indicative mood, which is used for statements of fact, such as He speaks English.
Note that the author uses the "mandative subjunctive" in
and my voice be still
Here's an explanation, again from Wikipedia:
The main use of the English present subjunctive, called the mandative or jussive subjunctive,[2] occurs in that clauses (declarative content clauses; the word that is sometimes omitted in informal and conversational usage) expressing a circumstance which is desired, demanded, recommended, necessary, or similar.