The Bible is the only source of Divine Revelation and is the only authoritative reference point of all things spiritual. Christians are required to read/hear the Bible, study the Bible, memorise the Bible, meditate on the Bible in its own pure, non-annotated form. The Bible is completely sufficient all by itself.
However, this does not mean that the Bible is the only resource for believers. God has provided believers with many different non-Bible resources to grow us in our faith. Granted, all non-Bible resources ought to be thoroughly Biblical in every way, but that does not nullify the instructions and allusions in Scripture to non-inspired-by-God resources for Christian maturity.
Take for example preaching. More than 90% of a sermon’s content is the preacher’s own words. If only one verse is preached with only a small handful of cross-references, the percentage of non-Bible content might be even greater. Yet, preaching is God’s way of saving sinners and sanctifying saints (2 Tim 4:2). A Christian with his Bible is sufficient for maturity, but God has also designed that a Christian be ministered to by a pastor preaching the Bible. Biblical preaching is preaching that not only reads the Bible words, but also explains its meaning for the sake of spiritual growth.
So also singing. Although a few Christians do believe that the “psalms, hymns, and spiritual songs” in Colossians 3:16 and Ephesians 5:19 are actual, and exclusive, Bible verses, one would be hard-pressed to prove that from Scripture. Yet, as Colossians 3:16 requires, these songs must originate from an internal fullness of “the words of Christ”. Singing is a non-Bible way of admonishing one another with the Bible.
And then, even in the New Testament era already, non-Bible, but still Biblical, Christian books and letter were a God-intended resource for Christians to grow in their understanding of God and what God requires of them. Paul mentions two non-Bible letters written to the Corinthians that he requires of the saints to remember (1 Cor 5:9 and 2 Cor 7:8). The inspired letter to the Colossians mentions a non-inspired letter to the Laodiceans that Paul wanted the Colossians to read (Col 4:16). Paul asked for reading material for himself that could well have been parts of the OT Bible, but probably included some other writings too (2 Tim 4:13).
We can add the admonitions, encouragements, and counsel of your fellow-saints and elders, not to mention letters and insights from believers and pastors who have gone before us in Church History. God has blessed us with the Bible, and many other aids for understanding and applying the Bible to our lives.
Test, with the Bible, every spiritual input in your soul. The Bible alone is the source of Divine Truth. But relish in the different ways in which God brings His Word to bear on on your life. Exploit the many MP3, Christian songs, and Christian books available for your spiritual growth. Discard all that speak their own wisdom; value all that are thoroughly Biblical.