I am assuming you are taking a signal from an earphone socket, into an external amplifier?
I have done this when I used to be a Disc Jockey, in the days of vinyl and tape decks... to add a tape deck to my system... the only work round you might need to consider is matching the 'responce' of the signals. Tape descks used a 'flat' responce. Any compedent electronics person, should be able to tell you the impedence differencial between the source and input, if you give them both ratings. It can be 'matched' using simple resistors.