Ping Pong Pi - A Ping Pong score and serving manager
Line by line analysis

import os


You need 2 blank lines before each global function definition.

def talk(n):


n is not descriptive and associated with a number. Don't use one letter variables unless it's really obvious (for example i loop counter, k and v as dict key and a value, or d as Twisted's deferred object. Something like phrase would be better.

    if voice_mode == 1:


Where does the voice_mode come from? Global variables are bad, pas it as a parameter to a talk method. Also, canonical way of checking if boolean is True is simply if voice_mode.

        os.system("espeak -ven+m3 '" + n + "' 2&gt;/dev/null")
    else:
        print n

def print_score(n):


Again, n does not tell reader a lot.

    if n == 0:
        other_n = 1


What is other_n? If there's no better name for it - add a comment.

    else:
        other_n = 0
    if d[n][1] == 0:


Where did d come from? Pass it to a function. Also, better name.

        score1 = "love"
    else:
        score1 = str(d[n][1])
    if d[other_n][1] == 0:
        score2 = "love"
    else:
        score2 = str(d[other_n][1])
    talk(score1 + " " + score2)

def win(n):


Same thing with the n variable. Also, docstring might be helpful for this method.

    global first_server


Global variables are bad, really bad. Make your method take and return this if you need the variable.

    talk(d[n][0] + " wins the game")


This d is really quite obscure.

    d[n][2] += 1
    d[0][1] = 0
    d[1][1] = 0
    deuce_mode = 0
    serve_turns = old_serve_turns
    if first_server == 0:
        first_server = 1
        current_server = 1
    else:
        first_server = 0
        current_server = 0
    if d[n][2] == match_points:
        talk(d[n][0] + " wins the match")
        d[0][2] = 0
        d[1][2] = 0

def add_score(n):


Same thing: add docstring, replace n with better name and pass all the global variables as arguments to the function.

    global deuce_mode
    global current_server
    global serve_turns
    d[n][1] += 1
    if n == 0:
        other_n = 1
    else:
        other_n = 0
    if deuce_mode == 0:
        if d[0][1] == d[1][1] and d[0][1] == game_points - 1:
            deuce_mode = 1
            old_serve_turns = serve_turns


I don't see old_serve_turns used anywhere.

            serve_turns = 1
        if current_server == 0:
            print_score(0)
        else:
            print_score(1)
        if d[n][1] == game_points:
            win(n)
    else:
        if d[n][1] == d[other_n][1] + 2:
            if current_server == 0:
                print_score(0)
            else:
                print_score(1)
            win(n)
        else:
            if current_server == 0:
                print_score(0)
            else:
                print_score(1)
    if (d[0][1] + d[1][1]) % serve_turns == 0:
        if current_server == 0:
            current_server = 1
            talk(d[1][0] + " is serving next")
        else:
            current_server = 0
            talk(d[0][0] + " is serving next")

d = []


You need either a better data structure, or at least a good comment explained what the d is. Also, d needs a better name.

for i in xrange(2):
    d.append(["", 0, 0])
d[0][0] = "Player 1"
d[1][0] = "Player 2"
game_points = 11
match_points = 2
deuce_mode = 0
voice_mode = 1
serve_turns = 2
old_serve_turns = 2
current_server = 2


You only play with server 0 and 1, correct? Why do you need to obscure and add server 2 which is only there to start the game? Better have a flag of some sort.

first_server = 0

talk("Ready to play")
while True:
    c = raw_input("Command: ").lower()


Better name: command.

    if c == "z":
        if current_server != 2:
            add_score(0)
        else:
            talk(d[0][0] + " is serving first")
            current_server = 0
            first_server = 0
    elif c == "x":
        if current_server != 2:
            add_score(1)
        else:
            talk(d[1][0] + " is serving first")
            current_server = 1
            first_server = 1
    elif c == "1name":
        d[0][0] = raw_input("Player 1 name: ")
        talk("Welcome, " + d[0][0])
    elif c == "2name":
        d[1][0] = raw_input("Player 2 name: ")
        talk("Welcome, " + d[1][0])
    elif c == "gm":
        game_points = int(raw_input("Game points: "))
        match_points = int(raw_input("Match points: "))
    elif c == "s":
        serve_turns = int(raw_input("Serve turns: "))
    elif c == "togglevoice":
        if voice_mode == 0:
            voice_mode = 1
        else:
            voice_mode = 0
    elif c == "quit":
        quit()
    else:
        talk("Command not recognised")


Even though this if/else tree is acceptable in your scope, you might want to look into Strategy pattern. This is further reading though.

Summary

Overall, your variable naming needs to be more readable. Don't use global variables, just pass the ones you need as arguments. For example, your talk function could be changed like this:

def talk(phrase, voice_mode=True):
    """Pronounce the phrase if the voice mode is on, otherwise print."""
    if voice_mode:
        os.system("espeak -ven+m3 '%s' 2&gt;/dev/null" % phrase)
    else:
        print phrase


Don't forget to specify docstrings for non-trivial methods. Make them long if you need to. Use comments extensively to elaborate if variable name or a data structure is not descriptive enough.