As per my knowledge I dont think that VLC playe can be converted in to Sinhala Language.It supports fix number of standard and mostly preferred languages and it doesnt includes Sinhala.
As per my knowledge the amr can be sucessfully converted to mp3 with a help of script on command line.With that you can easily and quickly carry out the conversion.
Here is a solution to your problem that may be the conversion is not perfect so it doesnt plays both audio and video or comapatibility problem with the player.Check that out surely you will get the solution.
Here is a solution for a problem that may be the DVD disk is not of good quality so it will take more time than natural or may be something wrong with the DVD writer itself.So check that out it will surely help.