Try this
https://ip_of_dmp:7777/set_param?sigma.rvol=50
https://ip_of_dmp:7777/set_param?sigma.lvol=50
 
You should probably save the settings too so add mib.save=1 to the end of it.
All together something like
 
https://ip_of_dmp:7777/set_param?sigma.rvol=50&sigma.lvol=50&mib.save=1
 
You can get a list of the MIBs to set here
https://ip_of_dmp:7777/get_param?p=*.*
 
Not sure what you mean about Channel? 
Are you talking about cast?