Good question: and I here give my thoughts -- not so much a Scriptural basis - but my experience and thinking . . .
First of all, if one would say that only prayers prayed while kneeling, or only audible prayers, etc. can/will be heard by God -- then this would in any situation rule out certain people as being unable to pray:
Example: The dumb person (unable to talk) is ruled out if prayer must be audible
The person with no legs would have a very difficult time if prayer had to be kneeling
Well, then, the objection might be that God could/would make an exception for the dumb person: but if He can hear the dumb person's inaudible prayer, then He can also hear my inaudible prayer!
Having said that, I think that there is extreme value in posture in prayer: both for ourselves - being prostrate before God shows reverence/awe/respect; and for others - as in the case of Daniel praying - it was a testimony to others.