These audio messages such as "I want to buy this product" or "I will give this product five stars review" are added at very low volume. A listener is not able to recognize them but his subconscious mind does and it produces an urge to buy the product. Is it allowed to use such messages to boost your sales?
That sounds like manipulation/brainwashing, and also extremely dishonest. Would you like it if someone did that to you? Made you think something without you knowing, something you wouldn't have otherwise?
A verse from the Quran regarding dishonesty:
“Woe to those that deal in fraud, – those who, when they have to receive by measure from men, exact full measure, but when they have to give by measure or weight to men, give less than due. Do they not think that they will be called to account- on a Mighty Day when (all) mankind will stand before the Lord of the Worlds.” (83: 1-6)
Aisha reported: There was no behavior more hateful to the Messenger of Allah, peace and blessings be upon him, than dishonesty. A man would tell a lie when speaking in the presence of the Prophet and he would not be satisfied until he knew that he had repented.
Source: Sunan al-Tirmidhī 1973 Grade: Sahih (authentic) according to Al-Albani
Hadith regarding dishonesty in business:
Similarly the Prophet SAWS (peace and blessings of Allaah be upon him) cautioned us against cheating and issued a warning to the one who does this. The Prophet SAWS (peace and blessings of Allaah be upon him) passed by a pile of food and put his hand into it, and his fingers touched something wet. He said, “What is this, O seller of the food?” The man said, “It got rained on, O Messenger of Allaah.” He said, “Why did you not put it (the wet part) on top of the pile so that the people could see it? He who deceives does not belong to me.” According to other reports, “He who deceives us is not one of us,” or, “He is not one of us who deceives us.” (Sahih Muslim 102).
Since the matter of using these subliminal messages to target the subconscious mind may be doubtful (doubtful to us regarding if it is 100% allowed):
This hadeeth was narrated by at-Tirmidhi (2442), Ahmad (1630) and Ibn Hibbaan (722) from al-Hasan ibn ‘Ali (may Allah be pleased with him) who said: I memorized from the Messenger of Allah (blessings and peace of Allah be upon him): “Leave that which makes you doubt for that which does not make you doubt, for truth leads to reassurance and lies lead to uncertainty.”
Classed as saheeh by Shaykh al-Albaani (may Allah have mercy on him).
Leave this act for the sake of Allah swt, to earn His pleasure, and Allah swt will grant you even more success in your business as well as the akhira, Ameen.