I have included notes where appropriate.
The second is the 'a' sound in words like 'map' and 'hand.'
Flap not thy hand like that.
So which is which? To most folks, the two sounds seem alike in their native dialect. There is a general rule, but it's pretty complicated: make the first change (as in 'master'), when the 'a' comes before 'f,' 's,' 'th,' and 'n,' unless the 'n' is followed by a 'd,' as in 'hand.' See? I told you it was complicated. To make things worse, there are a few exceptions, such as 'command.' This rule is only designed to get you through until your ear gets used to hearing these sounds.
Notice the difference between 'do' and 'due.'
Finally, here's a line to practice with. I recommend using this line to get into dialect before rehearsal or performance.
After many a year, it became necessary to bury the little old duke again.
And there you have it, my Lords and Ladies. All the sound changes necessary to speak like a gentleman. I wish you much joy of your Upper Class Dialect.