ipod dock upgrade for inmotion speakers unit
Does it just not work at all, or does it just not charge?

In order to get audio out of the dock connector you have to have an apple security chip that authenticates with the iDevice before the iDevice will output any audio. It may be that the older devices that you talk about do not require this chip while newer do, or that the chip has changed. Either way, it is beyond hobbyist level to hack it if the security chip is the issue. In fact it is difficult for even the experience cryptographers.

However, if it is an issue of it just not charging, then there is a hack that can be done with resistors on the data lines of the connector. I am pretty sure there is a question around here going over that, but I can not find a link to it at the moment.