Perhaps you have installed some SPA5xxG IP phones and have noticed that the audio sounds "hollow" or "quieter", or "bass-ier", or just plain different to what you have become used to using other phones.
Don't worry, you don't have faulty hardware. No, you've not discovered a bug either. :-)
Here's a paraphrased explanation from Cisco Engineering:
"It's just that most people are not used to the wideband audio that the handset is capable of delivering. Wideband provides more of the low end frequencies, it's what you would normally hear if you weren't on the phone. It's a similar listening experience to listening to FM instead of AM radio.
Wideband audio actually sounds more natural, but historically, telephony has been limited to narrowband audio, so that is what we are all used to hearing."
The SPA5xxG IP phones have enhanced acoustics to support G.722 wideband audio. Because of the enhanced acoustics, the SPA5xxG phones produce deeper bass than some people prefer. We're adding an option to reduce the bass, scheduled for release late July.
Once the release is available, you'll be able to do the following:
Disable via provisioning by changing
Or: Disable with the web-ui by changing
Voice tab > User tab > Audio Volume > Deep Bass: yes
Voice tab > User tab > Audio Volume > Deep Bass: no