Aux volume help!!?

You need to make sure the XM radio audio out is set high. Some XM radios vary the output with the XM radio's volume control. Some XM radios can be optioned to have a preset audio output. It depends on the model.

But first you should verify the AUX IN was installed properly by connecting it to a known good source like a phone that works well when connected elsewhere. If the known good phone audio with it's volume up doesn't give you good volume through the Prius system, then go back to the AUX IN installer. Since it sounds like you had the aux in adapter installed for the XM, you might have that installer check the whole setup realizing the XM itself might need configuration.

Update: A quick search for "2005 Prius Aux In" suggests the best way to add an AUX IN is through a box connected to the back of the Prius radio that emulates a CD changer. Other "solutions" include FM modulators that take the Aux audio and transmit it to an unused FM frequency. Some are transmitted through the air to your antenna, others fit between the car's antenna cable and the factory radio. Both FM techniques work but the first (through the air) has more interference and is not a recommended solution (even though it's the cheapest) because a radio station can override your signal..
How to connect my phone to my 2005 Prius -- no audio jack - carstereo | Ask MetaFilter

GROM Adapters - see $75 Aux In only unit
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That is a FM radio modulator, probably the least expensive kind that goes through the air to your antenna. Make sure the XM radio has a high output and also tune that channel WITHOUT the FM modulator plugged in. Hopefully there is no station that you can hear on that frequency. IF there is, the over the air FM modulators can usually be changed to another frequency but often just a few, 3 or 4. In the end if it doesn't work right, the other two methods I listed above would be better. But you would have to get your money back at Best Buy and find an independent audio shop to install the other options.

 

It's worth noting that some of the better options could allow bluetooth connectivity with your phone as well as the Aux In for your XM to allow handsfree phone calls and music that would interrupt the XM when needed, just like the newer cars with all this built in.