proclaim his salvation day after day.
Does it get any more clear than this?
Q1. Who do you sing to? A: the LORD
Q2. Who should sing to the LORD? A: all the earth
Q3. What should we sing? A: proclaim his salvation (e.g. tell people how God has saved them from sin, death, and the devil)
Q4. How often should we sing? A: day after dayAny questions?