A. The translation of ex aequo demulcens will have to takes its cue from mentem acuat, and to a lesser extent 'exercises...' 'delights...' 'teaches...' Perhaps translate as
'stimulates the mind...- equally soothing' (the soul, eyes, ears)
'excites the mind...- equally calming'
B. concentu carminum etc for a modern audience draws attention to all ...
omnium elegantissima: The form of omnium is genitive plural, dependent on the superlative: 'most elegant of all.'
acuat…exerceat…oblectet…doceat: Note that these 4 verbs are subjunctive. The construction could be the so-called relative clause of characteristic, which is generalizing and can be used to talk about the characteristics of a broad class: 'that ...