Adverbs do not show concordance as adjectives do. Nouns have gender and adjectives have gender concordance. Bajo would be called a masculine adjective form while baja would be called a feminine adjective form (meaning short). As a adverb it is always bajo. (masculine, singular)
Yo hablo bajo. (if yo is a male or female) (or bajito) being an adverb.
Yo hablo en voz baja (baja being an adjective here..feminine to match voz)
en voz baja=in a soft or low voice
exception: I shouldn't even mention this as it will probably confuse you:
Adverbs in a series
If you are using a series of adverbs that end in -mente, the -mente typically is retained on only the final adverb in the series:
- Lo explicó sincera y cariñosamente. He explained it sincerely and lovingly.
- Puede instalar nuestro software fácil, segura y rápidamente. You can install our software easily, safely and quickly.
Note that in these series, the adjective-as-adverb is in the singular feminine form, as it would be if the -mente were retained.