Well dear ricpiano,
Hard to describe "round".
(Just fell on some side information of this Arabesque) The piano, the favourite instrument of Debussy has a very rooted position in the musical production of Claude Debussy. It's precisely with this instrument that he will introduce a new language, a conception of new music, inspired by the "Orient" and at the same time by his interest for the sonorous phenomenon. Debussy composed his two Arabesques in 1888, and both posess these refinements and a very 'modern' spirit.
Here is some general information on how to play this Arabesque which you already play very well. So just take this information for what it is, maybe it does not all apply to your interpretation.
Try to create a stable climat in the left hand (do not let it 'hang') and make the right hand sing. Now, if you conduct the middle section very smoothly, the lyrical temperament of the superior part will come out 'naturally'. What matters is to create a climate, an atmosphere and this... naturally. That's not easy. Measure 3: articulate a little more the white notes in the left hand. Do not exaggerate articulation, try to articulate 'close'. You best start to study the left hand, try to feel totally at ease with the left hand in order that you can focus your concentration on the right hand. Measure 11: Be careful with pedalling in the parts where the right hand is written in bound (tied) notes. Measure 26: Make the do# sound "round" ('timbré" in French) in order to maintain the continuity of the melody (pianissimo). Same with the fa# in measure 28. Avoid heaviness in the arpeggios. You can play in measure 35 the left hand 'portato', with fore-arm. Now for the 'tempo rubato' in measure 39, play a little slower. Think at a choral and try to give music to the polyphony and harmony in this passage. Feel free ! Take time for certain chords but give the time back in others. It's all a question of rhythmical balance, give and take. But still focus on your right hand, the melody. Measure 63; it's the only part where you have to look for a firm sound. Think at winds, hobo, clarinette, bassoon, horn... Measure 95, try to study this bit extremely piano and slow, try to have rhythm and sound 'egal'. Measure 103; focus on continuity; no gaps... as if this part was played by only one hand.
Good luck !
Kindly