T&B has the Energy Substitution feat which allows you to pick an energy type. You can now use this on a spell that causes energy damage, substituting in the feat's energy type.
Example: Cast your Cone of Cold with Energy Substitution: Sonic and it does 1d6/level Sonic damage instead of Cold damage.
The feat doesn't modify the spell level.
Compare that with the Shout spell (also 4th level) that only does 2d6 sonic damage and causes deafness. Against a very limited number of creatures (crystaline creatures, brittle or crystaline objects) it would do a full 1d6/level, but that is much more of a special case.
Looking through the Monster Manual, we could not find a creature Immune to Sonic attacks. There are plenty of creatures immune to the other energy types, but we couldn't find an example immune to sonic damage.
In play, is this choice of feats unbalanced? The feat seems fine for the other energy types, it is just sonic that is suspect.
Example: Cast your Cone of Cold with Energy Substitution: Sonic and it does 1d6/level Sonic damage instead of Cold damage.
The feat doesn't modify the spell level.
Compare that with the Shout spell (also 4th level) that only does 2d6 sonic damage and causes deafness. Against a very limited number of creatures (crystaline creatures, brittle or crystaline objects) it would do a full 1d6/level, but that is much more of a special case.
Looking through the Monster Manual, we could not find a creature Immune to Sonic attacks. There are plenty of creatures immune to the other energy types, but we couldn't find an example immune to sonic damage.
In play, is this choice of feats unbalanced? The feat seems fine for the other energy types, it is just sonic that is suspect.