The original article written in Tamil :- http://thamizhan-thiravidana.blogspot.in/2011/08/67-1.html (67. தமிழ் மொழியின் முந்தைய பெயர் -1)
वाक्यज्ञम् मधुरैः वाक्यैः स्नेह युक्तम् अरिन्दम ||
तम् तम् अर्थम् अभिप्रेत्य ययौवाक्यम् उदीरयन् (2-49-18)
Normally people translate this as : She spoke words serenely and spoke sweetly’.
तम् वानर इन्द्रम् मधुर अर्थम् उक्त्वा (5-36-31)
उवाच वत्स माम् विद्धि वयस्यम् पितुर् आत्मनः (3-14-3).
Similarly in Sundhara Khanda, Dadhimukha complained to Sugreeva that the vanaras headed by Angada and Hanuman had destroyed the Madhuvana and beaten him and other security guards of the Madhuvana. Dadhimukha was in tears and in pain from the beatings, while he conveyed the news to Sugreeva. Sugreeva in turn conveyed this news to Rama and Lakshmana who were with him at that time. But how did he convey? What did he actually say? He said that Dadhimikha had come to tell in "Madhura Vaag" (मधुर वाग्) the news of the destruction of the Madhuvana by the vanaras!
एतत् अर्थम् अयम् प्राप्तो वक्तुम् मधुर वाग् इह (5-63-21)
Would anyone beaten up well and fled from the scene to inform his superior of the atrocity tell the news in "sweet words"?
Again in Uttara Khanda, just before shedding his mortal coils, Rama gives a piece of advice to Vibhishana in "madhuraṃ ślakṣṇayā girā" (7- 98-21)
विभीषणम अथॊवाच मधुरं शलक्ष्णया गिरा
Everyone was stupefied at the developments of Rama planning to leave the earth. And there was no Madhuram in the words of Rama spoken to various people. But on seeing Vibhishana who had come to see him on hearing the developments, Rama used Madhuram speech! The selective use of Madhuram in the context of talk with Vibhishana can at best be about speaking in the language that Vibhishana was conversant with.