As for uttering the intention in prayer, it is controversial issue among the Muslim scholars and you can apply the view which seems convenient to your situations.
Dr. `Abdul-Fattah `Ashoor, Professor of the Exegesis of the Qur’an at Al-Azhar University, issues the following Fatwa: “As you have mentioned in your question, the intention should be made in the heart. As regards making an audible intention, there are different opinions among Muslim jurists that maybe clarified as follows:
Imam Abu Hanifah maintains that uttering the intention is an act of bid`ah or innovation in religion. That is because neither the Messenger of Allah (peace and blessings be upon him) nor his Companions (may Allah be pleased with them all) were reported to have enunciated the intention. However, the Hanafi scholars prefer uttering the intention to ward off Satanic insinuation in prayer.
As for the Malikis, they are of the opinion that pronouncing the intention runs counter to that opinion which is deemed to be the most correct except when the person thinks that he will most probably suffer from Satanic insinuation. In this case, uttering the intention is strongly recommended.
The Shafi`is and some other scholars hold the view that it is recommendable to pronounce the
intention by the tongue in order to help the heart.
So, if the praying person pronounced the intention but his/her tongue uttered other than what he/she has intended in his/her heart, the intention is still valid and acceptable.
Dear brother, you can choose from the aforementioned opinions what you deem proper in your case. However, if you could not make the intention properly and you feel that you are obsessed concerning it, I advise you to utter it and to specify how many prayer units do you intend to pray. For example, you may say, “I intend to observe the Morning prayer, two obligatory prayer units on time and in congregation or following this Imam and then you say ‘Allahu Akbar’ (Allah is the Greatest).
In other cases, you are recommended to make the intention by your heart taking into consideration that you have come to the Mosque only to perform the Morning Prayer, for example, in congregation. In this case, you are to pronounce the first saying ‘Allahu Akbar’ in silence.