Without JP1, I think the only solution is to learn the vol+ and vol- keys from the original remote into the 15-1994.

The learned keys won't punch through, so if you need TV vol in more than TV mode you need to relearn the VOL keys in each device mode that needs them.

With JP1, you could create a device upgrade which has the same device and command numbers as TV/0060 but has the correct NEC characteristics. (Probably someone has already posted that upgrade, because this question has been asked before).

When you learn the signals (non JP1 solution) make sure you hold the orriginal remote VOL key for a long time during the learn process, so the 15-1994 has a chance to see the correct repeat characteristics of the signal.