Does anybody know what is the purpose of "AUX" port on Cisco 7940 IP phone?
I see, your problem is simply that large ringtones don't play on the 7940.
You have to consider that most cisco phones are different from each other. Different hardware, different software. In particular the 7911 is a different generation as well, being a java-based phone, unlike the 7940. So it's not surprising that is has more memory.
So you will have to limit ringfiles for the 7940, to about 75KB as you've found.
On the 796x only, to connect the expansion module 7914. On both 794x and 796x, you can connect a magnetic card reader, or other RS-232 device. This require use of third-party software.
Hope this helps, please rate post if it does!