This is the 5793rd most frequent Japanese word.
のっ
"のっ" is not a standalone word in Japanese; it may be part of a longer word, phrase, or an informal/emotive expression. Context is needed for definition.
Here, "の" is used to connect "写っている" (shown in) and "は" (denotes the subject), forming "写っているの", which introduces the subject depicted.
写真に写っているのは私です。
The person shown in the picture is me.
In this case, "の" serves as a possessive particle connecting "ピアノ" (piano) and "音" (sound), indicating the sound produced by the piano.
昨日彼が演奏したピアノの音が素晴らしかった。
The sound of the piano he played yesterday was amazing.
Here, "の" is part of the phrase "明るいの", nominalizing "明るい" (bright) to treat it as a noun-like construct in context.