Hi,
You don't mention file type but I am assuming .avi as your media file. I think external .srt files streamed to your TV will not be recogized or playable with either the built in UPNP server or almost any other that you could "addin" but I may be wrong. You might try a USB stick with an avi and srt in the same folder directly connected to your TV just to see if it will play that way. But you want it streamed anyway, so, editing your .avi files might be a method. AVIADDXSub910 is a software which can be used to add subs to .avi files and is supposed to be pretty fast. There are other possible softwares HANDBRAKE is another great program which can get the job done but will take longer. There are other softwares which I am sure can do what you want. The 2 mentioned are free. Good luck