General Antonyms:
* Harmonious
* Concordant
* Consonant
* Agreeable
* Compatible
* Unified
* Cohesive
* Symphonic
* Melodic
* Consistent
More Specific Antonyms, Depending on Context:
* Peaceful (if discordant refers to conflict)
* United (if discordant refers to a lack of unity)
* Pleasant (if discordant refers to unpleasant sounds)
* Balanced (if discordant refers to a lack of balance)
The best antonym will depend on the specific context in which you are using the word "discordant."