Let me get this straight: you have media on your pc and other devices that you would like to play back on your N900.
What is not clear is whether you want to be able to control the playback on your N900.

There are two options I see available:
1. Lower-tech:
Treat yourself to a short-range fmtx (do a web search for one, they're cheap), which you can plug into your computer or hi-fi. Then, pick up this signal on your N900 with one of the radio applications.
*The drawbacks:
- The radio rx on the N900 requires the headphones to be plugged in. because they are the ariel. Therefore, I suspect you will need to do something software-wise if you wish to output audio to the speakers.
- It won't be possible to control the media.
- Don't know what range of tx you will need to suit your requirements.
*Advantages
- if it''s ok to listen using headphones, it's very easy.
- media will run and you can tune in/out as you wish.
- if you require speakers as well, and sending sound to headphones AND speakers is allowed by N900's subsystem, then doing so should be quite easy by the quick alteration of a conf file (I have no idea which though).

2. Higher-tech
Using a server-client setup, stream media from your pc to your N900 via your wireless network. The stream can either be continuous or controlable - think of an on-demand radio service (e.g. BBC iPlayer radio), where programmes can either be listened to live or as the user desires. I am not a good enough programmer with php/similar but I'm sure something is possible - it may exist already? What is Knots, can it work in reverse?
* Downsides
- It'll take a while to sort out the software, even if it exists because normally one plays the media on the larger device.
- If nothing suitable exists both software and an interface will need to be developed.

* Upsides
- You will be able to play music (and video I suppose) throughout your network.
- Quality of output will be better than fm because a lot less interference.
- Depending on seup and interface, you may be able to control media as well as play a live stream.

Which of these best fits your needs - the fm or stream solution, or neither because I misunderstood the question?