HoldAll cannot be the whole story, because the following works:

expressionForm = Cos[2 Pi hz seconds];
Block[{hz = 800}, Table[expressionForm, {seconds, 0, 2}]]
Block[{hz = 800}, Plot[expressionForm, {seconds, 0, 2}]]

although Table and Plot both have the HoldAll attribute.

Thanks very much for the references and the example.

Thanks for your clear response.

Your solution with "/." is good for my specific example, but I'm left curious why Block doesn't work in my attempt (as an exercise while I'm trying to learn the Wolfram language.) Block does work with an arbitrary function in place of Play:

In[9]:= Block[{hz = 300}, f[expressionForm, {seconds, 0, 2}]]

Out[9]= f[Cos[600 \[Pi] seconds], {seconds, 0, 2}]

Block also works with the function-operator Integrate:

In[11]:= Block[{hz = 300}, Integrate[expressionForm, seconds]]

Out[11]= Sin[600 \[Pi] seconds]/(600 \[Pi])

My previous success using Block is why I was surprised when it didn't work with Play. Ideally I'd like to be able to predict ahead of time when I need to use "/." instead of Block, but I haven't yet grasped the principle at play here.