I have a few options for you. (All definitions from the Oxford Dictionaries)
Concord is a noun that means to be in agreement or in harmony.
formal Agreement or harmony between people or groups.
‘a pact of peace and concord’
1.1 [count noun] A treaty.
‘a concord was to be drawn up’
Grammar Agreement between words in gender, number, case, person, or any other grammatical category which affects the forms of the words.
[count noun] A chord that is pleasing or satisfactory in itself.
There is an adjective form, concordant:
In agreement; consistent.
‘the answers were roughly concordant’
1.1 Music In harmony.
Related is consonance:
Agreement or compatibility between opinions or actions.
‘consonance between conservation measures and existing agricultural practice’
‘a constitution in consonance with the people's customs’
1.1 The recurrence of similar-sounding consonants in close proximity, especially in prosody.
‘the abrupt quality of the sound is echoed in the final ‘t’ consonance of ‘discreet’ and ‘shut’’
1.2 [Music] A combination of notes which are in harmony with each other due to the relationship between their frequencies.
‘all music creates tension and release, dissonance and consonance’
[count noun] ‘the tendency to place strong consonances in lower parts’
The adjective form of consonance is consonant, but you have to be careful that folks don't think you're talking about the letters that aren't vowels.
attributive Denoting or relating to a consonant.
‘a consonant phoneme’
[consonant with] In agreement or harmony with.
‘the findings are consonant with other research’
2.1 Music Making a harmonious interval or chord.
‘the bass is consonant with all the upper notes’
Lastly, there is unison which can apparently be used as an adjective (though I've never seen it used that way):
Simultaneous performance or utterance of action or speech.
‘‘Yes, sir,’ said the girls in unison’
Coincidence in pitch of sounds or notes.
‘the flutes play in unison with the violas’
2.1 [count noun] A combination of notes, voices, or instruments at the same pitch or (especially when singing) in octaves.
‘good unisons are formed by flutes, oboes, and clarinets’
Performed in unison.
‘unison congregational singing’