Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy:
Why lov'st thou that which thou receiv'st not gladly
Or else receiv'st with pleasure thine annoy?
If the true concord of well-tuned sounds
By unions married, do offend thine ear
They do but sweetly chide thee, who confounds
In singleness the parts that thou shouldst bear
Mark how one string, sweet husband to another
Strikes each in each by mutual ordering;
Resembling sire and child and happy mother
Who, all in one, one pleasing note do sing:
The next  quatrain  draws an analogy between marriage and a musical instrument that is correctly tuned, so that the strings are in harmony or “sweet husband to another”. Together the strings resemble a family; father, mother and child. In Shakespeare’s time “sire” meant “father” — both a verb and a noun. 
 Note that the adjective “sweet” echoes those in lines two and seven. 
 In Elizabethan times roles were believed to be divinely ordained, hence the expression “mutual ordering”.  Unless single life was chosen in service of God, the expectation was that people would marry and children follow. 
 
 The three quatrains end with a colon which forms the  volta  or turn, followed by a  caesura . The final couplet follows.
Sonnet 8