The hymn now extends the prayer it has just began with another prayer that I think is also lost in imagery.
Streams of mercy, never ceasing, call for songs of loudest praise
This is more straightforward. The streams of mercy are the blessings that we get from Christ–the Fountain. Streams of mercy which never cease. May our meditation on these mercies stir our hearts to praise Him loudly. The streams call for songs of loudest praise.
The prayer continues with perhaps more difficult imagery:
Teach me some melodious sonnet sung by flaming tongues above,
Praise the mount, I’m fixed upon it, mount of Thy redeeming love.
The prayer is still addressed to Christ, the fountain of very blessing. We are still asking Him to help us to sing. We want Him to prime our hearts to be consumed with Him and all His merciful deeds for us. We want to be aflame for Him. If He does this to us, we can sing as we should.
And which people have a richer, fuller understanding of God’s mercy more than the believers who are now in heaven, with Christ? And so the hymn prays to Christ to teach us the melodious song or sonnet that the tounges above, aflame with Christ, sing.
They see Him and His deeds more clearly. They sing better than us, not because their throats are free from from the effects of the fall, but because their eyes are. They sing to One they’re seeing.
We now vow to remain on this one place, this one topic -the love of God. We will never move on from the fact that God has redeemed us through Christ. In our pilgrimage, our journey on earth, we’ve come to this one mountain. We have come to the knowledge of God’s love, which indeed is a mountain.
And we are now ‘fixed upon it’. We don’t move on from the mountain that is God’s redeeming love. Praise this mount (possibly the hymnist means Calvary). Here we will build an altar of stone as a memorial to God who has helped us to get to this place. We here raise our ‘Ebenezer’, as Samuel did saying “the Lord has helped us” (see 2 Sam. 7:12). But then we’ll be going to verse 2 of the hymn.
Hopefully this tunes your heart, by the grace of Christ, to sing this hymn next time you hear it, with loudest praise!