The ATtinyX4 family has a "USI" (Universal Serial Interface). BroHogan has provided this for the ATtinyX5 family...
http://code.google.com/p/arduino-tiny/wiki/TWIMasterLibrary
USI is identical for the two processor families. In other words, ATtinyX4 processors have some hardware support for I2C and you will very likely be able to find the software you need via Google.
Full hardware PWM is out. The processor has two timers / four outputs. One timer is needed to generate the tone leaving one timer / two outputs for PWM. I suspect you will get good results with software PWM.
11 I/O pins available. Two I/O pins need for I2C, three for LEDs, one for motor, one for buzzer for a total of seven needed.
For what it's worth, the last time I priced ATtiny24s, ATtiny44s were cheaper.
Looks to me like an ATtinyX4 would work very well.