Praise be to Allaah.
Beautifying the voice when reciting Qur'aan is something mustahabb that is encouraged in many texts, such as the words of the Prophet (blessings and peace of Allah be upon him): Adorn the Qur'aan with your voices. Narrated by al-Nasaa'i, 1015; Abu Dawood, 1468; classed as saheeh by al-Albaani.
i.e., make your voices beautiful when reciting, because beautiful words become even more beautiful when recited in a beautiful voice, and this is something well known.
End quote from Haashiyat Sunan al-Nasaa'i, 2/179
By doing it in a beautiful voice and doing it well, this encourages people to listen to it and ponder it.
End quote from Fayd al-Qadeer, 4/90.
Being keen to beautify one's voice when reciting Qur'aan is something that is prescribed in Islam, and it is not regarded as blameworthy effort or showing off.
This is indicated by the report narrated from Abu Moosa al-Ashari (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) listened to my recitation at night. The next morning he said: O Abu Moosa, I listened to your recitation last night; you have been given a beautiful voice like the beautiful voice of Dawood. I said: O Messenger of Allah, if I had known you were there, I would have made it more beautiful?
Narrated by al-Nasaa'i in al-Sunan al-Kubra, 7/273; Ibn Hibbaan in his Saheeh, 16/169; al-Bayhaqi in his Sunan, 3/12; classed as saheeh by al-Albaani (3532).
Al-Haafiz Ibn Katheer said: This indicates that it is permissible to put extra effort into beautifying one's voice. Abu Moosa had been given a beautiful voice along with perfect focus, and the gentleness of the people of Yemen. This indicates that this is something that is acceptable in shareeah.
End quote from Tafseer Ibn Katheer, 1/63.
Shaykh Muhammad ibn Saalih al-Uthaymeen (may Allah have mercy on him) said:
This indicates that if a person makes his voice beautiful when reciting Qur'aan so that the listener may enjoy it and feel happy when hearing it, there is nothing wrong with that and it is not regarded as showing off. Rather this encourages people to listen to the word of Allah, may He be glorified and exalted, so that the people will feel happy to hear it.
End quote from Sharh Riyadh al-Saaliheen, 4/662
Al-Haafiz said in al-Fath (9/63):
There is no doubt that people are more inclined to listen to recitation if it is done in a melodious voice than to listen to one who does not have a melodious voice, because the melodious voice is more effective in softening the heart and bringing tears.
What may be understood from the evidence is that beautifying the voice when reciting Qur'aan is something desirable; if a person's voice is not beautiful than he should beautify it as much as he can, as Ibn Abi Mulaykah said. End quote.
And he said: Ibn Abi Dawood narrated via Ibn Abi Musjiah who said: Umar used to give precedence to a young man with a beautiful voice because of his beautiful voice (to lead people in prayer).
End quote from Fath al-Baari, 9/80
If your intention by beautifying the voice is to achieve the hoped-for aims of shareeah, such as obtaining proper focus and humility in the heart and making the eye weep, or to move the people when listening to the Qur'aan, this is something that is prescribed in Islam and is encouraged, and it cannot be regarded as showing off at all.
But if your aim in doing that is to show how beautiful your voice is and how good your recitation is so that people will praise you for it, then this is a kind of showing off and you have to strive against these thoughts so that your action will be sincerely for the sake of Allah alone.
See also the answer to questions no. 9359 and 156796.
And Allah knows best.