Why does ear wax taste sour?
Earwax, also called cerumen, is slightly acidic (1), with a pH of about 6,  and acidic foods or substances taste sour. The composition of earwax, upon which its taste depends, is related to its functions. Earwax aids in cleaning and lubrication of the ear canal and has an antimicrobial effect. The antimicrobial effect is in part attributed to its acidity, which might give an evolutionary reason for the sour taste.

From Wikipedia:


  Cerumen is produced in the outer third of the cartilaginous portion of
  the ear canal. It is a mixture of viscous secretions from sebaceous
  glands and less-viscous ones from modified apocrine sweat glands.[4]
  The primary components of earwax are shed layers of skin, with 60% of
  the earwax consisting of keratin, 12–20% saturated and unsaturated
  long-chain fatty acids, alcohols, squalene and 6–9% cholesterol.[5]
  
  (...)
  
  While studies conducted up until the 1960s found little evidence
  supporting antibacterial activity for cerumen,[15] more recent studies
  have found that cerumen has a bactericidal effect on some strains of
  bacteria. These antimicrobial properties are due principally to the
  presence of saturated fatty acids, lysozyme and, especially, to the
  slight acidity of cerumen.