I think you are confusing things, just a little bit.
PHP = Server side programming language
Ajax, JavaScript, jQuery..etc = Client side programming languages.
After reading everything I guess you want ajax to check remaining time OnPageLoad and then make a countdown to know when to refresh. That is possible, but will only update for that client. So you don't really have to call anything.
You just need to read the text file and check last saved information about the song
And then you will do:
Total time - current time = remaining time.
And set a page refresh in the remaining time. But you have to care about something. If you are streaming (which I guess you are doing) a total page refresh will also restart the stream, and also the stream has some delay so maybe it will not be totally synced. (Which is not bad at all for the information part, because if it updates 1 sec later...it's ok. But if you refresh the page and the user has to stream all over again for each song...that will suck). So yea, please message me, I will make a temp bid and provide me more information.
I have totally understood what you want to do but I really want to check few things first before making a bid. Regards.