There are Youtube sources, as well as some CD's and downloads but it would be very hard to produce a standard version for all to use.
In addition to pronunciation there are also variations in the wording used.
I don't know if my experience is common, but I've always understood that mantra pronunciation is transmitted from guru to disciple to ensure correctness, and that it is placed within the relevant context.
I know when I look on Youtube or listen to mp3 files that I find variations which are different from that given to me by my lamas.
Having said all that, it would be a wonderful resource if all of the mantras were recorded by a representative of each of the main schools as an archive for reference and download. For some mantras, evidence of having received the relevant empowerment may be required, as it is now by some online sources of sadhanas such as the FPMT.
This site is my favourite mantra site:http://www.visiblemantra.org/index.html
One wonderful component in any archive must be that of Lama Tashi's Master Chants.
(I believe he is HHDL's own chanting teacher):http://www.youtube.com/watch?v=oML5UU3jekY
I could immerse myself in these chants for hours.