General Synonyms:
* Dull: Lacking interest or excitement
* Uninteresting: Not engaging or stimulating
* Bland: Lacking strong features or characteristics
* Tedious: Boring and tiresome
* Repetitive: Occurring again and again in a way that is boring
* Unvarying: Always the same; not changing
* Uniform: Having the same form or appearance throughout
* Flat: Lacking in emotional intensity or variation
* Boring: Not interesting or stimulating
* Uninspired: Lacking in originality or creativity
Synonyms related to sound:
* Monotonous: Characterized by a single, unchanging tone or pitch
* Uninflected: Spoken in a voice that lacks inflection
* Dreary: Lacking in cheerfulness or excitement
* Droning: Speaking in a low, monotonous tone
* Humdrum: Lacking in excitement or interest
* Tiresome: Causing boredom or weariness
Synonyms related to visual aspects:
* Uniform: Having the same form or appearance throughout
* Unvarying: Always the same; not changing
* Plain: Simple and lacking in decoration
* Uninspired: Lacking in originality or creativity
The best synonym will depend on the specific context in which you are using the word.