You didn't say what are you playing but i assume a .mp3
One way you can do like this.
I have added some test mp3 with name:
laz_1.mp3
laz_2.mp3
laz_3.mp3
laz_4.mp3
And the code goes like this (I commented it for you)
Sub random_Click
Dim i As Int
i = Rnd(1,4) ' random number from 1 to 4
mediaplayer1.Load(File.DirAssets, "laz_"&i&".mp3") ' load our mp3 with random number
Timer1.Initialize("timer1", 1000)
MediaPlayer1.Play ' media is playing
timer1.Enabled = True
ToastMessageShow("Now is playing laz_"&i&".mp3", False) ' just check to see what is playing
End Sub