I think the problem is that he wants to use ent_playloop or ent_playsound for a VERY LONG entity (e.g. a river).
He could place some dummy entities along the river - each of them playing an ambient sound. But then the sound will not be constant when walking along the riverside.

I think the problem is:
When using ent_playsound or ent_playloop the audio source is point-like, and can not be a whole surface.