“If the true concord of well-tuned sounds, By unions married”
'Concord' is a technical music term for notes that sound pleasing together, as opposed to discord. 'By unions married' means the strings are joined together like a married couple — their harmony is a kind of wedlock. The second quatrain argues that if even this musical joining offends the youth, the music is rebuking him for failing to create the same union through marriage and children.
“who confounds In singleness the parts that thou shouldst bear”
'Confounds' means ruins or destroys. 'Singleness' is the youth's unmarried state, but in music it means a single note sounding alone rather than in a chord. 'Parts' are both personal duties (husband, father) and the separate melodic lines that combine in part-singing. The line says simultaneously: you ruin the musical harmony by playing a solo, and you ruin your social duties by staying single.
historical The procreation argument (Sonnets 1-17)“Mark how one string, sweet husband to another, Strikes each in each by mutual ordering; Resembling sire and child and happy mother”
'Mark' means 'pay attention' — the poet directs the youth to observe a lute or viol string that vibrates in sympathy with the one beside it. This is an extended simile: a string is a 'sweet husband' to the next string, and when they sound together they resemble a father, a mother, and a child all singing in harmony. The poem converts the instrument itself into a model family to shame the youth into following suit.
historical The procreation argument (Sonnets 1-17)