Wow, this sure sounds like a VLC issue, it seems like it's failing to transcode audio sometimes. Can you edit the N900 profile and test different audio codecs, change audio format from mp3 to mpga and save the profile and test again with just the client. You can try mp2a, mp2, mp4a instead of mp3. Let me know if any of those produce sound for you when you try with just the client.

Edit: Another thing you could test is to change the VLC port range start setting from 19780 to 19781. If you get sound only when there are two streams, it's possible that your firewall doesn't allow connection to port 19780 but 19781 works.