This is a very common situation unfortunately there is little legal recourse. In these cases, neighborliness is much more effective.
-Michael R. Juarez Law Office of Juarez and Schaeffer PO Box 16216 San Diego, CA 92105 (619) 804-4327 www.jslaw.org Mike@jslaw.org This posting is provided for “information purposes” only and should not be relied upon as "legal advice." Nothing transmitted from this posting constitutes the establishment of an attorney-client relationship. Applicability of the legal principles discussed here may differ substantially in individual situations or in different jurisdictions.
You may have rights. I have been involved in construction defect lawsuits where the HOA sued the developer for failing to design and construct the flooring system to provide a sufficient sound attenuation level. Building codes provide the acceptable levels. Also, I am aware of HOA's where the CC&R's (the governing documents) prohibit 2nd level unit owners from installing hardwood floors without proving they will install sound-attenuating underlayment. So, you should first look at your CC&R's to see whether this issue is addressed. If not, and your project is less than ten years old, you should explore with the Board of Directors whether a demand to the original builder is warranted. If you are beyond 10 years since the Notice of Completion was recorded on your project, you should heed Atty. Juarez's advice and buy your upstairs neighbor an area rug.
First thing I wonder is whether the HOA rules allow for hardwood floor on second floor. Sometimes they do, sometimes they don't. Sometimes HOA decides to change the rules based on enough complaints. But, usually those who already have hardwood floor would be grandfathered.