Are you using a loop to continously check if it's play time (08:00 in your example)?
Use a service instead. Start it at the desidered time, hold a lock and play the audio. Then release the lock and wait for next "wake up" time.
Or if it's for a running app, just use a timer ticking each minute to check if it's time to play the audio.
Dim Timer1 As Timer ' in Process_Globals
..
'Activity_Create
Timer1.Initialize("Timer1",1000 * 60) 'check each minute
Timer1.Enabled = True 'start timer
...
Sub Timer1_Tick
If DateTime.Time (DateTime.Now) = "08:00" AND Not(mp.IsPlaying) Then
Timer1.Enabled = False
mp.Play
End If
End Sub
File/export as zip then publish your code here so others could check about possible errors in it.
BTW, "it does not work !" is not that helpful as a message..