Singing a song with one dynamic makes for a very boring song.
Consider how you would speak the text of the song as if telling a story to a small child? Would you vary the volume? Of course you would otherwise the child would soon become bored. This is the same for an audience if songs are not delivered with varying dynamics.
Singing both loudly and softly requires the same amount of effort (breath support) with the only difference being your thought process and the amount of air leaving your lungs. Many try to sing quieter by not supporting the breath and all that does is is produce a weak sound that tends to go flat. Some singers slow down when they are singing quietly. A quiet sound should be just as rounded and strong as a loud sound and requires effort.
The easiest way to sing quieter is to imagine singing to somebody standing very close to you, you want them to be able to hear the text clearly, so clear diction is needed, but you don’t want to have others hear you. The less air that escapes your body will make the sound quieter, in order to do this support is needed with a relaxed throat. You must have an adequate space in your mouth so avoid thinking that reducing the space will help with quieter singing.
In order to sing loud the air flow must increase, use good support and increase the space in your mouth without creating any tension. As you increase the airflow it must be supported by the abdominal muscles as otherwise there will be too much pressure on the vocal chords and they will crack and not produce the louder sound desired.
Some songs call for a gradual increase/decrease in volume. This requires a clear idea of what it is you want to do and keeping your support mechanism engaged.
It is easier to sing high notes loudly than low notes the same is true that singing low notes quietly is easier than singing high notes quietly.
Exercise
Be aware of what it is you are doing when you sing loudly/softly. Try to listen to yourself, the easiest way to do that if you are struggling to do so in a group is to cup your hand behind one of your ears.
