Sound and mind are two separate words, each having its own unique meaning. But sound does influence our thoughts. However, interestingly in English the two together “sound mind” means your mind is functional in the natural human society. The word sound takes on another meaning: rational; able to make reasonably acceptable decision; functional. All are relative to the human society as a functional entity for this world to continue its increasingly precarious existence on planet earth.
But there are deeper meaning on the spiritual realm. here are two references from the Bible. The Biblical sound mind is a spiritual mind, which functions perfectly.
In the following first case, Apostle Paul specified that Biblical sound mind means not regarding the things of God, including God’s people, according to our natural mind, but according to our spiritual mind. He also assured that we are not the mere physical natural man anymore. We have become a new creation (in the spiritual realm). So we can have a spiritual mind, and do function more superior using this new mind.
2 Corinthians 5:12-14, 16-17 (NKJV) Be Reconciled to God : 12., …that you may have an answer for those who boast in appearance and not in heart. 13 For if we are beside ourselves, it is for God; or if we are of sound mind, it is for you…then all died;…16 Therefore, from now on, we regard no one according to the flesh. Even though we have known Christ according to the flesh, yet now we know Him thus no longer. 17 Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.
In the following second case, Apostle Paul elaborated further this sound mind. Take note that he associated sound mind with a number of crucial words and phrases: gift of God, spirit, power and love, not be ashamed of Jesus Christ, hold fast the pattern of sound words, in faith and love in Christ Jesus, and being kept by the Holy Spirit who dwells in us.
2 Timothy 1:6-9; 13-14 (NKJV): 6 Therefore I remind you to stir up the gift of God which is in you through the laying on of my hands. 7 For God has not given us a spirit of fear, but of power and of love and of a sound mind. 8 Therefore do not be ashamed of the testimony of our Lord, nor of me His prisoner, but share with me in the sufferings for the gospel according to the power of God, 9 who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was given to us in Christ Jesus before time began,…13 Hold fast the pattern of sound words which you have heard from me, in faith and love which are in Christ Jesus. 14 That good thing which was committed to you, keep by the Holy Spirit who dwells in us.
The best mind ever is the mind of Christ. 1 Corinthians 2:15-16 15 But he who is spiritual judges all things, yet he himself is rightly judged by no one. 16 For “who has known the mind of the Lord that he may instruct Him?” But we have the mind of Christ.