Declaration from marriage registrar himself is also not valid?
Is it mandatory to have a registered marriage certificate for Visa approval?
Please go through below link :
http://travel.state.gov/content/visas/english/fees/reciprocity-by-country/IN.html
Kindly suggest.

The link the following information

http://travel.state.gov/content/visas/english/fees/reciprocity-by-country/IN.html

Marriage Certificates
Available. The Hindu and Muslim communities do not usually register marriages, however, marriages by Hindus, Buddhists, Jains or Sikhs may be voluntarily registered under the Hindu Marriage Act of l955. This Act does not apply to Muslims, Parsis, Jews or Christians, who may register their marriages under the Special Marriage Act of l954, or the Christian Marriage Act. Marriage certificates for marriages registered under these Acts may be obtained from the offices of Government Registrar of Marriages, which are located in the headquarters of each district. The certificate will be issued by the Registrar only if the bride and groom personally appear before the official and pay the required fee.

A certificate of marriage between Muslims is usually issued by the priest who performed the ceremony. The document is in the Urdu language, and a certified translation is required. Marriages between Christians are usually obtainable from Church records. If the marriage has not been officially registered, then two sworn affidavits giving the names, dates and places of birth of the bride and groom, and the date and place of marriage, as well as the names of the parents of both parties are acceptable. The affidavits must be executed by one of the parents of each party, or if the parents are deceased, by the nearest relative of each party who was present at the wedding.

Note: A document termed as "Marriage Agreement" or "Deed of Marriage" to live as man and wife (under the Registration Act of l908) is not confirmation of a marriage solemnized legally under the Indian Marriage Acts now in force. Such a document does not confer upon the contracting parties' legal marital status under the law.

Same-sex marriages are not legally recognized in India.

As per above content , the unregistered marriage certificate we have from marriage registrar himself with the stamps of district registrar office and Hindu marriage registrar plus affidavits from our parents should be good enough for my marriage proof . Kindly suggest if I'm missing something vital here .

We cannot register as my Husband is in the US and it is hard to fly to India now.

Thank you everyone . Really appreciate the quick response . Now I'm less worried than before . 
I have decided to carry the unregistered document along with affidavits from my parents regarding the marriage for my interview . Hoping to get through the process . Fingers crossed ????