'Where are we going now' can suggest:
1. a previous arrangement has been made.
'Where do we go now can suggest:
2. In such situations as this, what do we normally do?
3. What is on the scheduled programme at this point?
Note that it is rather difficult to find a real difference in meaning between #1 and #3. Course books and student grammars that try to explain the difference between the various tenses and aspects generally do a reasonably good job, but there are situations in which two or more tenses/aspects could be appropriate. The one chosen just happens to fit what is in the speaker's moment at the time of uttering it.
There is also the point that song writers are often more interested in the flow of sounds than in the pedant's view of what tense should be used in a classroom situation.