When one speaks to a guest or any other person in a gathering or alone, one should always maintain a pleasant voice, with a low but audible tone. Raising the voice is contrary to correct adab and indicates lack of respect for the person to whom you are talking. One should maintain this adab whether at home or in any other place, with family friends and others. Adab starts at home. If you only observe something outside of the home and not outside then this is clearly only for show.
Perhaps the most important point to note is to adhere to this adab when speaking with one's parents or someone of status or an elder or someone whom you have great respect for. If appropriate one should smile whilst talking to others (obviously not to non-mahrams!). It is is the simple action such as the smile that may act as your charity or simply allow others to be more receptive towards what you have to say, dispelling the stern and humourless impression many have of Muslims today.
The Quran relates to us the advice of Luqman the sage to his son, directing him to speak in a gentle manner, speaking loudly is detested and ugly.
وَٱقۡصِدۡ فِى مَشۡيِكَ وَٱغۡضُضۡ مِن صَوۡتِكَۚ إِنَّ أَنكَرَ ٱلۡأَصۡوَٲتِ لَصَوۡتُ ٱلۡحَمِيرِ
"And be moderate in thy pace, and lower thy voice; for the harshest of sounds without doubt is the braying of the ass."
(Surah Luqman: verse 19)
Similarly in verses two and three of Surah Hujarat:
يَـٰٓأَيُّہَا ٱلَّذِينَ ءَامَنُواْ لَا تَرۡفَعُوٓاْ أَصۡوَٲتَكُمۡ فَوۡقَ صَوۡتِ ٱلنَّبِىِّ وَلَا تَجۡهَرُواْ لَهُ ۥ بِٱلۡقَوۡلِ كَجَهۡرِ بَعۡضِڪُمۡ لِبَعۡضٍ أَن تَحۡبَطَ أَعۡمَـٰلُكُمۡ وَأَنتُمۡ لَا
تَشۡعُرُونَ - إِنَّ ٱلَّذِينَ يَغُضُّونَ أَصۡوَٲتَهُمۡ عِندَ رَسُولِ ٱللَّهِ أُوْلَـٰٓٮِٕكَ ٱلَّذِينَ ٱمۡتَحَنَ ٱللَّهُ قُلُوبَہُمۡ لِلتَّقۡوَىٰۚ لَهُم مَّغۡفِرَةٌ۬ وَأَجۡرٌ عَظِيمٌ
O ye who believe! Raise not your voices above the voice of the Prophet, nor speak aloud to him in talk, as ye may speak aloud to one another lest your deeds become vain and ye perceive not. (2) Those that lower their voice in the presence of Allah's Messenger― their hearts has Allah tested for piety: for them is Forgiveness and a great Reward. (3)
Imam Bukhari (رحمة الله عليه) reported that 'Abdullah ibn al-Zubayr (رضى الله عنه) said that after the revelation of this verse, whenever Umar ibn al-Khattab (رضى الله عنه) wanted to speak to the Prophet (صلى الله عليه وسلم), he would talk almost in whispers and the Prophet (صلى الله عليه وسلم) could hardly hear him and would ask him to repeat what he said (Sahih Bukhari).
Al Hafiz al-Dhahabi wrote in his biography of Imam ibn Sireen (رحمة الله عليه) the great scholar, "Whenever he was in his mother's presence, he would talk in such a hushed voice that you would think he was ill." (Tarikh al-Islam)
In his biography of Abdullah ibn Awn al-Basri (رحمة الله عليه), a student of Imam Ibn Sireen (رحمة الله عليه) and one of the famous scholars, he noted:
"One time his mother called him and because he responded with a voice louder than hers, he was fearful and repentant, thus freeing two slaves." (Ibid)
So it is important to only talk as loud as is necessary, and not to raise ones voice over the one whom one respects or is honourable.
Ettiquette between brothers and sisters in conversation
It may ofte be necessary, particularly in the west for a non related brother and sister to speak, whether it be in a shop, at school, college, a hospital, or otherwise. Talking reflects ones personality, this can often cause a problem in conversations between sisters and brothers who are non-mahram to each other. When a sister talks with a non-mahram brother, she should be aware of her tone. She must not beautify her voice but attempt to make it thicker with the intention of pleasing Allah and not falling prey to the one who may have illness in his heart. This means she has to make more effort if her voice is particularly gentle, so inshaAllah will have a greater reward for it. This is not something one should overlook, particularly given the times we live in.
Another problem one faces today is the western ettiquette of "looking at the person you are talking to". This is another cause for concern, as the commands of Allah are being shelved and brothers and sisters believe it is okay to "look" because you are talking to that person. One should ask oneself, does the blind man see the people he talks to in order to judge their tone or understand what they are saying? It is simply unnecessary, and one should take care of ones tone and lower one's gaze when talking to members of the opposite sex, do not give shaytaan a free invitation.