I too have noticed this problem since Day 1. I filed a report using the Apple Bug Reporter on 26/10/11 as follows:
Summary: When using a custom ringtone, the audio quality is bad in comparison to Text tones.
Steps to Reproduce: Place a custom ringtone on the device and select it as a ringtone AND text tone. Then note the difference in audio quality between when a text alert is heard and when a call alert is heard.
Expected Results: Ringtone quality should be as good as text tone quality
Actual Results: Ringtone quality is poor and tinny, Text tone quality is good and bassy
Regression: New in iOS 5.0/iPhone 4S, this issue was NOT present in previous builds
Notes: Attached a custom .m4r where the quality difference is very noticeable.
I received a reply to this stating that the bug had already been reported:
Hello Frank,
This is a follow up to Bug ID# 10346850. After further investigation it has been determined that this is a known issue, which is currently being investigated by engineering. This issue has been filed in our bug database under the original Bug ID# 10344733. The original bug number being used to track this duplicate issue can be found in the State column, in this format: Duplicate/OrigBug#.
Thank you for submitting this bug report. We truly appreciate your assistance in helping us discover and isolate bugs.
Best Regards,
Apple Developer Connection
After logging in and checking on it, I can see that the status for this Bug ID is "Closed". I can only assume that it means it's been fixed. But as of iOS 5.0.1 (9A406), this is still a very noticeable issue.
I have replied twice to ask if it has been fixed in an upcoming iOS release, but no one has responded 😟.
It's now been nearly 4 months! Hope it's fixed soon as it's borderline embarassing hearing my phone ring and watching everyone else cover their ears!! Doesn't create a good impression for other people when they see what phone I have!!